feat(3-K): real geocoder backends + producer-doc reframe + consumer-doc enrichment

Second of three PRs for v0.5.0 (J shipped the framework; this fills in real
backends + documents the reframed design principle in-tree; L is the events
tab + map fix, then tag).

Backends (all satisfy GeocoderBackend; never raise, all-null on any failure):
- NaviBackend — composed Navi /api/reverse/<lat>/<lon> (name/address + timezone
  + landclass + elevation in one call). Near-passthrough: response already
  matches the canonical 9-field shape. Best-effort warmup ping (Boise) on
  construction when a loop is running; config `headers` slot for a future
  Authorization: Bearer (config-only, no code change). Default base_url
  http://192.168.1.130:8440.
- PhotonBackend — raw Photon /reverse?lat&lon&limit=1 (name/address only).
  Maps features[0].properties; postal_code <- postcode; timezone/landclass/
  elevation_m null (Navi-composed-endpoint extras).
- NominatimBackend — OSM Nominatim /reverse?format=jsonv2 (name/address only).
  Configurable rate limit (default 1/sec; 0 disables for self-hosted) +
  required User-Agent. Maps the address block; landclass/elevation_m/timezone
  null.

Registered all three in supervisor _BACKEND_REGISTRY (resolved by EnrichmentConfig
backend_class name).

Docs — design pivot now in-tree:
- PRODUCER §2 reframed: the verbatim Matt quote stays; the translation inverts.
  Central is the consumer's only data plane (consumers can't do follow-up
  lookups), so enrich deliberately and centrally, namespaced under _enriched,
  failing to null. "No enrichment" is gone.
- PRODUCER §10.1 inverted: enrichment is expected; the anti-pattern is doing it
  OUTSIDE the framework (inline in poll(), bypassing cache + _enriched
  namespacing + the never-raise safety net).
- PRODUCER new §13 Enrichment contract: Enricher / GeocoderEnricher /
  GeocoderBackend Protocols, NoOpBackend default, sqlite cache + TTL +
  cache-all-null + don't-cache-on-raise semantics, _enriched.<name> provenance,
  per-field coverage matrix (cross-checked against GEOCODER_FIELDS), and the
  landclass antimeridian known wrinkle.
- CONSUMER FIRMS section: documents the data._enriched.geocoder bundle (9
  fields), per-region coverage (US-full, non-US timezone+elevation), and the
  antimeridian landclass caveat.

Tests:
- test_navi/photon/nominatim_backend.py — happy-path field mapping, null
  handling, extra-key drop, network/timeout/non-200/malformed -> all-null
  (never raises), Nominatim rate-limit (disabled + spacing) + User-Agent.
  Env-gated live Navi smoke (NAVI_INTEGRATION_TEST=1; skipped by default — the
  192.168.1.130 endpoint isn't reachable from CT104's segment).
- test_producer_doc.py — +4: §2 verbatim quote present, §10.1 subsection exists,
  §13 names all four protocol types, §13 coverage matrix == GEOCODER_FIELDS
  (derived from code, not hardcoded).

Verification: full pytest 525 passed, 1 skipped (was 495; +30 backend +
4 doc tests, -1 the env-gated skip). grep subject_for_event/_ADAPTER_REGISTRY
clean. All three backends import + resolve via the registry.

Flagged for later (NOT done here): adapters besides FIRMS that should declare
enrichment_locations (nwis, eonet, gdacs, usgs_quake, wfigs_*) — that's PR L
scope alongside the events tab. See PR description.

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

tests/test_nominatim_backend.py

@@ -0,0 +1,118 @@
+"""Tests for NominatimBackend (OSM Nominatim /reverse jsonv2)."""
+
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from central.enrichment.backends.nominatim import (
+    DEFAULT_USER_AGENT,
+    NominatimBackend,
+)
+from central.enrichment.geocoder import GEOCODER_FIELDS, all_null_bundle
+
+_NOMINATIM_OK = {
+    "name": "Boise",
+    "display_name": "Boise, Ada County, Idaho, United States",
+    "address": {
+        "city": "Boise",
+        "county": "Ada County",
+        "state": "Idaho",
+        "country": "United States",
+        "postcode": "83702",
+    },
+}
+
+
+def _backend(**kw) -> NominatimBackend:
+    kw.setdefault("base_url", "http://nominatim.test")
+    kw.setdefault("rate_limit_per_sec", 0)  # disabled by default in tests
+    return NominatimBackend(**kw)
+
+
+def test_url_and_format():
+    b = _backend()
+    url = b._url(43.6, -116.2)
+    assert url.startswith("http://nominatim.test/reverse?")
+    assert "format=jsonv2" in url and "lat=43.6" in url and "lon=-116.2" in url
+
+
+def test_user_agent_header_present():
+    b = _backend()
+    assert b._request_headers()["User-Agent"] == DEFAULT_USER_AGENT
+
+
+def test_custom_user_agent():
+    b = _backend(user_agent="myapp/1.0 (me@example.com)")
+    assert b._request_headers()["User-Agent"] == "myapp/1.0 (me@example.com)"
+
+
+@pytest.mark.asyncio
+async def test_happy_path_maps_address_block():
+    b = _backend()
+    b._fetch = AsyncMock(return_value=dict(_NOMINATIM_OK))
+    result = await b.reverse(43.6, -116.2)
+    assert set(result.keys()) == set(GEOCODER_FIELDS)
+    assert result["name"] == "Boise"
+    assert result["city"] == "Boise"
+    assert result["county"] == "Ada County"
+    assert result["state"] == "Idaho"
+    assert result["country"] == "United States"
+    assert result["postal_code"] == "83702"
+
+
+@pytest.mark.asyncio
+async def test_navi_only_fields_null():
+    b = _backend()
+    b._fetch = AsyncMock(return_value=dict(_NOMINATIM_OK))
+    result = await b.reverse(43.6, -116.2)
+    assert result["timezone"] is None
+    assert result["landclass"] is None
+    assert result["elevation_m"] is None
+
+
+@pytest.mark.asyncio
+async def test_city_falls_back_to_town_then_village():
+    b = _backend()
+    b._fetch = AsyncMock(return_value={"address": {"village": "Tinytown"}})
+    result = await b.reverse(1.0, 2.0)
+    assert result["city"] == "Tinytown"
+
+
+@pytest.mark.asyncio
+async def test_network_error_returns_all_null():
+    b = _backend()
+    b._fetch = AsyncMock(side_effect=ConnectionError("down"))
+    assert await b.reverse(1.0, 2.0) == all_null_bundle()
+
+
+@pytest.mark.asyncio
+async def test_malformed_json_returns_all_null():
+    b = _backend()
+    b._fetch = AsyncMock(side_effect=ValueError("bad"))
+    assert await b.reverse(1.0, 2.0) == all_null_bundle()
+
+
+@pytest.mark.asyncio
+async def test_rate_limit_disabled_does_not_sleep():
+    b = _backend(rate_limit_per_sec=0)
+    with patch("central.enrichment.backends.nominatim.asyncio.sleep") as slp:
+        await b._throttle()
+        await b._throttle()
+        slp.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_rate_limit_spaces_consecutive_requests():
+    """With a 1/sec limit, the second back-to-back call must sleep a positive
+    interval. Mock monotonic to a fixed instant so the throttle computes a wait."""
+    b = _backend(rate_limit_per_sec=1.0)
+    sleeps: list[float] = []
+
+    async def _fake_sleep(d):
+        sleeps.append(d)
+
+    with patch("central.enrichment.backends.nominatim.time.monotonic", return_value=100.0), \
+         patch("central.enrichment.backends.nominatim.asyncio.sleep", side_effect=_fake_sleep):
+        await b._throttle()  # first: last_request_at was 0 -> no wait
+        await b._throttle()  # second: now==100, last==100 -> wait ~1.0s
+    assert any(d > 0 for d in sleeps), f"expected a positive throttle sleep, got {sleeps}"