fix(3-L.5): per-backend settings schemas (fixes build_enrichers TypeError)

Surfaced during the 2026-05-20 NaviBackend activation: toggling
config.enrichment.backend_class to NoOpBackend while backend_settings still
held {"base_url": ...} crashed _rebuild_enrichers with
`TypeError: NoOpBackend() takes no arguments`, BEFORE invalidate() ran. Fixed
by mirroring the SourceAdapter.settings_schema pattern: each backend declares a
Pydantic settings_schema; validation happens at write-time (GUI POST) and
read-time (supervisor). A mismatch is now a clean ValidationError, never a
constructor TypeError.

Backends — each gets a `<Name>BackendSettings(BaseModel, extra="forbid")` +
`settings_schema` class attr, mirroring __init__ defaults EXACTLY (note:
timeout_s stays 10.0 — the brief's "5.0" was a transcription slip; preserve the
production default):
  NoOpBackend     -> NoOpBackendSettings        (no fields)
  NaviBackend     -> NaviBackendSettings        (base_url, timeout_s, headers, warmup)
  PhotonBackend   -> PhotonBackendSettings       (base_url, timeout_s, headers)
  NominatimBackend-> NominatimBackendSettings   (base_url, user_agent, rate_limit_per_sec, timeout_s)

GeocoderBackend Protocol (in geocoder.py, where the base actually lives — not
base.py, which only has Enricher) gains `settings_schema: type[BaseModel]`.

supervisor:
- build_enrichers validates backend_cls.settings_schema.model_validate(
  backend_settings) before instantiating, and constructs from the validated
  .model_dump(). ValidationError (not TypeError) on mismatch.
- _rebuild_enrichers builds into locals and commits to instance state only on
  success — a ValidationError leaves the previously-active enrichers/config/
  cache untouched.
- _handle_enrichment_change wraps the rebuild in try/except ValidationError:
  logs and returns, keeping the previous backend running (supervisor stays up;
  operator fixes the row; next NOTIFY applies cleanly). No cache invalidation
  on a failed change.

GUI /enrichment:
- GET skips the outer EnrichmentConfig.backend_settings field and renders a
  separate <fieldset> from describe_fields(backend_cls.settings_schema, ...)
  for the row's current backend_class. Backend fields namespaced bs_<name>.
- POST reassembles bs_<name> inputs into a backend_settings dict, validates it
  against the SUBMITTED backend_class's schema (so errors attach to the right
  fields when an operator is mid-switch), then validates the outer
  EnrichmentConfig. DB row written only if both pass; otherwise re-renders with
  field-level errors against the submitted backend.
- backend_class stays a plain text field (no <select>, no client-side reshape).

form_descriptors: generic `float -> "number"` widget (2 lines, mirrors K.5's
`dict -> "json"`), needed because backend schemas have float fields
(timeout_s, rate_limit_per_sec). Benefits any float field codebase-wide.

DB schema unchanged: backend_settings stays JSONB; validation moved to
use-site. _BACKEND_REGISTRY / _ENRICHER_REGISTRY unchanged beyond schema lookup.

Tests (test_backend_settings_schema.py, 11): schemas exist + extra='forbid';
Navi schema preserves defaults (timeout_s == 10.0); NoOp has zero fields;
build_enrichers raises ValidationError-not-TypeError for the exact 2026-05-20
case; supervisor keeps previous backend on a bad NOTIFY (the incident
scenario); valid NoOp-with-empty-settings applies + invalidates; GUI POST
rejects bad backend_settings without writing + re-renders against submitted
backend; GUI POST writes on valid settings. test_enrichment_config_plumbing
updated for the new context shape (outer_fields/backend_fields).

Verification: full pytest 546 passed, 1 skipped (was 535; +11). grep
subject_for_event/_ADAPTER_REGISTRY and grep 100.64.0./192.168.1. in src both
empty.

Does NOT touch PR L scope (events tab, remaining adapter enrichment_locations),
the DB schema, or the registries.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_backend_settings_schema.py

@@ -0,0 +1,235 @@
+"""Tests for per-backend settings schemas (PR L.5).
+
+Each GeocoderBackend declares a Pydantic settings_schema (extra='forbid').
+build_enrichers validates backend_settings against it BEFORE constructing, so
+a config/settings mismatch is a clean ValidationError, not a TypeError. The
+supervisor's hot-reload keeps the previous backend running on ValidationError;
+the GUI POST re-renders with field errors and does not write the DB row.
+
+Regression guard for the 2026-05-20 incident: switching backend_class to
+NoOpBackend while backend_settings still held {"base_url": ...} crashed
+_rebuild_enrichers with `TypeError: NoOpBackend() takes no arguments`.
+"""
+
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+from pydantic import BaseModel, ValidationError
+
+from central.config_models import EnrichmentConfig
+from central.enrichment.cache import EnrichmentCache
+from central.supervisor import _BACKEND_REGISTRY, build_enrichers
+
+
+# --- schemas exist + extra='forbid' -----------------------------------------
+
+def test_every_backend_declares_a_settings_schema():
+    for name, cls in _BACKEND_REGISTRY.items():
+        schema = getattr(cls, "settings_schema", None)
+        assert schema is not None, f"{name} has no settings_schema"
+        assert isinstance(schema, type) and issubclass(schema, BaseModel), name
+
+
+def test_every_settings_schema_forbids_extra():
+    for name, cls in _BACKEND_REGISTRY.items():
+        with pytest.raises(ValidationError):
+            cls.settings_schema.model_validate({"definitely_not_a_field": 1})
+
+
+def test_navi_schema_accepts_valid_kwargs_and_preserves_defaults():
+    from central.enrichment.backends.navi import NaviBackendSettings
+
+    m = NaviBackendSettings()
+    assert m.base_url == "http://localhost:8440"
+    assert m.timeout_s == 10.0  # preserved from __init__, NOT 5.0
+    assert m.headers is None
+    assert m.warmup is True
+    m2 = NaviBackendSettings(base_url="http://navi:8440", timeout_s=3.0, warmup=False)
+    assert m2.base_url == "http://navi:8440" and m2.timeout_s == 3.0
+
+
+def test_noop_schema_has_no_fields():
+    from central.enrichment.backends.no_op import NoOpBackendSettings
+
+    assert list(NoOpBackendSettings.model_fields) == []
+
+
+# --- build_enrichers validation ----------------------------------------------
+
+def _cache(tmp_path) -> EnrichmentCache:
+    return EnrichmentCache(tmp_path / "c.db")
+
+
+def test_build_enrichers_raises_validation_error_not_typeerror(tmp_path):
+    """The exact 2026-05-20 bug: NoOpBackend + stale base_url. Must be a clean
+    ValidationError, never a TypeError from the constructor."""
+    cfg = EnrichmentConfig(backend_class="NoOpBackend", backend_settings={"base_url": "http://x"})
+    with pytest.raises(ValidationError):
+        build_enrichers(cfg, _cache(tmp_path))
+
+
+def test_build_enrichers_navi_valid_settings(tmp_path):
+    from central.enrichment.backends.navi import NaviBackend
+
+    cfg = EnrichmentConfig(
+        backend_class="NaviBackend",
+        backend_settings={"base_url": "http://navi:8440", "warmup": False},
+    )
+    enrichers = build_enrichers(cfg, _cache(tmp_path))
+    assert isinstance(enrichers[0]._backend, NaviBackend)
+
+
+def test_build_enrichers_navi_unknown_setting_rejected(tmp_path):
+    cfg = EnrichmentConfig(
+        backend_class="NaviBackend",
+        backend_settings={"base_url": "http://navi:8440", "bogus": 1},
+    )
+    with pytest.raises(ValidationError):
+        build_enrichers(cfg, _cache(tmp_path))
+
+
+# --- supervisor hot-reload keeps previous backend on ValidationError ---------
+
+def _supervisor():
+    from central import supervisor as sup_mod
+
+    config_source = MagicMock()
+    config_source.get_enrichment_config = AsyncMock(return_value=EnrichmentConfig())
+    return sup_mod.Supervisor(
+        config_source=config_source,
+        config_store=MagicMock(),
+        nats_url="nats://localhost:4222",
+    )
+
+
+@pytest.mark.asyncio
+async def test_handle_enrichment_change_keeps_previous_on_validation_error():
+    """Navi active, then a bad NOTIFY (NoOp + leftover base_url) arrives. The
+    supervisor must keep NaviBackend running and not crash."""
+    from central.enrichment.backends.navi import NaviBackend
+    from central.enrichment.backends.no_op import NoOpBackend
+
+    with patch("central.supervisor.EnrichmentCache") as cache_cls:
+        cache_cls.return_value = MagicMock(invalidate=AsyncMock(return_value=0))
+        sup = _supervisor()
+        # Establish a valid NaviBackend active state.
+        sup._rebuild_enrichers(EnrichmentConfig(
+            backend_class="NaviBackend",
+            backend_settings={"base_url": "http://navi:8440", "warmup": False},
+        ))
+        assert isinstance(sup._enrichers[0]._backend, NaviBackend)
+        active_before = sup._active_enrichment_config
+
+        # Bad config arrives via NOTIFY: NoOp + stale base_url -> ValidationError.
+        sup._config_source.get_enrichment_config = AsyncMock(
+            return_value=EnrichmentConfig(
+                backend_class="NoOpBackend",
+                backend_settings={"base_url": "http://navi:8440"},
+            )
+        )
+        await sup._handle_enrichment_change()  # must NOT raise
+
+        # Previous backend still active; config unchanged; cache NOT invalidated.
+        assert isinstance(sup._enrichers[0]._backend, NaviBackend)
+        assert sup._active_enrichment_config is active_before
+        cache_cls.return_value.invalidate.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_handle_enrichment_change_applies_valid_noop_with_empty_settings():
+    """Switching to NoOp with empty backend_settings (the correct way) applies
+    cleanly and invalidates the cache."""
+    from central.enrichment.backends.no_op import NoOpBackend
+
+    with patch("central.supervisor.EnrichmentCache") as cache_cls:
+        cache_cls.return_value = MagicMock(invalidate=AsyncMock(return_value=3))
+        sup = _supervisor()
+        sup._rebuild_enrichers(EnrichmentConfig(
+            backend_class="NaviBackend",
+            backend_settings={"base_url": "http://navi:8440", "warmup": False},
+        ))
+        sup._config_source.get_enrichment_config = AsyncMock(
+            return_value=EnrichmentConfig(backend_class="NoOpBackend", backend_settings={})
+        )
+        await sup._handle_enrichment_change()
+        assert isinstance(sup._enrichers[0]._backend, NoOpBackend)
+        cache_cls.return_value.invalidate.assert_awaited()
+
+
+# --- GUI POST validates backend_settings + does not write on error ----------
+
+def _mock_pool(conn):
+    pool = MagicMock()
+    cm = MagicMock()
+    cm.__aenter__ = AsyncMock(return_value=conn)
+    cm.__aexit__ = AsyncMock(return_value=None)
+    pool.acquire = MagicMock(return_value=cm)
+    return pool
+
+
+@pytest.mark.asyncio
+async def test_gui_post_rejects_bad_backend_settings_without_writing():
+    """POST with a bad backend setting (timeout_s non-numeric) re-renders with a
+    field error keyed bs_timeout_s and does NOT execute the DB upsert."""
+    from central.gui.routes import enrichment_update
+
+    request = MagicMock()
+    request.state.operator = MagicMock(username="op")
+    request.state.csrf_token = "tok"
+
+    form = {
+        "csrf_token": "tok",
+        "enricher_class": "GeocoderEnricher",
+        "backend_class": "NaviBackend",
+        "cache_ttl_s": "86400",
+        "bs_base_url": "http://navi:8440",
+        "bs_timeout_s": "not-a-number",
+    }
+    request.form = AsyncMock(return_value=form)
+
+    conn = MagicMock()
+    conn.execute = AsyncMock()
+    conn.fetchrow = AsyncMock(return_value={})
+    templates = MagicMock()
+    templates.TemplateResponse.return_value = MagicMock()
+
+    with patch("central.gui.routes._get_templates", return_value=templates), \
+         patch("central.gui.routes.get_pool", return_value=_mock_pool(conn)):
+        await enrichment_update(request)
+
+    conn.execute.assert_not_awaited()  # no DB write on validation failure
+    ctx = templates.TemplateResponse.call_args.kwargs["context"]
+    assert "bs_timeout_s" in ctx["errors"]
+    assert ctx["backend_class"] == "NaviBackend"  # re-rendered against SUBMITTED backend
+
+
+@pytest.mark.asyncio
+async def test_gui_post_writes_on_valid_settings():
+    from central.gui.routes import enrichment_update
+
+    request = MagicMock()
+    request.state.operator = MagicMock(username="op")
+    request.state.csrf_token = "tok"
+    form = {
+        "csrf_token": "tok",
+        "enricher_class": "GeocoderEnricher",
+        "backend_class": "NaviBackend",
+        "cache_ttl_s": "86400",
+        "bs_base_url": "http://navi:8440",
+        "bs_timeout_s": "10",
+    }
+    request.form = AsyncMock(return_value=form)
+    conn = MagicMock()
+    conn.execute = AsyncMock()
+    templates = MagicMock()
+
+    with patch("central.gui.routes._get_templates", return_value=templates), \
+         patch("central.gui.routes.get_pool", return_value=_mock_pool(conn)):
+        resp = await enrichment_update(request)
+
+    conn.execute.assert_awaited()  # DB upsert ran
+    # the backend_settings written are the validated/dumped dict
+    args = conn.execute.call_args.args
+    assert "INSERT INTO config.enrichment" in args[0]
+    assert resp.status_code == 302