feat(2-G.5): preview_for_settings framework hook + NWIS opt-in

Adds an optional async hook on SourceAdapter so any adapter can surface a
settings-driven preview on its /adapters/<name> edit page. The framework
renders the result generically as a table — no adapter-name branches in
GUI templates or route code.

Framework changes:
- src/central/adapter.py: new async preview_for_settings(self, settings)
  on the base class, default returns None. Adapters opt in by overriding;
  non-overriding adapters render unchanged.
- src/central/gui/routes.py: GET /adapters/{name} instantiates the adapter
  with a no-op _PreviewConfigStore stub and a /dev/null cursor path (GUI
  has no live ConfigStore), constructs settings_obj via the schema, and
  calls preview_for_settings inside a try/except. Result lands in template
  context as preview_rows / preview_error.
- src/central/gui/templates/_adapter_preview.html: new partial. Generic
  table with columns derived from the first dict's keys; error banner
  mirrors the existing last_error article style.
- src/central/gui/templates/adapters_edit.html: one-line include between
  the Region fieldset and Save/Cancel.

NWIS opt-in:
- New NWIS_MONITORING_LOCATIONS_URL constant and _PREVIEW_LIMIT cap of 50.
- preview_for_settings returns None when region is None, otherwise one-shot
  fetches monitoring-locations within the bbox via a fresh aiohttp session.
  Must work even when adapter is not started -- the GUI process never calls
  startup(). Returns list[dict] with the contract column order: site_id,
  name, site_type, state. Errors propagate so the framework can render the
  operator-visible banner.
- HTTP call factored into _fetch_preview_text so tests mock cleanly.

Tests (7 new):
- tests/test_preview_hook.py: default returns None; partial renders list
  with correct headers/rows/count; partial renders error banner; partial
  renders empty when both context values are None.
- tests/test_nwis.py adds TestNWISPreview: returns None without region,
  returns rows with correct column order, propagates HTTP errors.

Verification:
- 457/457 full suite green (was 450; +7 new tests).
- Live /adapters/nwis preview returns 50 rows with the contract keys
  against the current production Iowa bbox.
- /adapters/eonet preview_for_settings returns None via base default --
  proves framework is duck-typed, no NWIS-specific code in framework.

src/central/gui/routes.py

@@ -46,6 +46,9 @@ from central.gui.audit import (
 )
 from functools import cache
 
+from pathlib import Path
+
+from central.config_models import AdapterConfig
 from central.gui.db import get_pool
 from central.gui.form_descriptors import describe_fields, FieldDescriptor
 from central.adapter_discovery import discover_adapters
@@ -55,13 +58,23 @@ from pydantic import ValidationError
 @cache
 def _adapter_classes() -> dict:
     """Cached adapter class discovery.
-    
+
     GUI is a separate process from supervisor; walks pkgutil itself.
     Python's import cache makes subsequent calls free.
     """
     return discover_adapters()
 
 
+class _PreviewConfigStore:
+    """No-op stand-in passed to adapter __init__ when calling preview_for_settings.
+
+    preview_for_settings implementations must create their own one-shot HTTP
+    session and must not depend on config_store / cursor_db state — the GUI
+    process has no live ConfigStore (the supervisor owns the real one)."""
+
+    pass
+
+
 router = APIRouter()
 
 # Streams to display on dashboard -- derived from the registry's dashboard flag.
@@ -1450,6 +1463,28 @@ async def adapters_edit_form(
             )
             api_key_missing = not has_key
 
+    # Generic settings-driven preview. Adapters opt in by overriding
+    # SourceAdapter.preview_for_settings; the framework is duck-typed on the
+    # returned list[dict] shape and never branches on adapter name.
+    preview_rows: list[dict] | None = None
+    preview_error: str | None = None
+    if adapter_cls is not None and hasattr(adapter_cls, "settings_schema"):
+        try:
+            settings_obj = adapter_cls.settings_schema(**settings)
+            preview_cfg = AdapterConfig(
+                name=row["name"],
+                enabled=row["enabled"],
+                cadence_s=row["cadence_s"],
+                settings=settings,
+                updated_at=row["updated_at"],
+            )
+            preview_adapter = adapter_cls(
+                preview_cfg, _PreviewConfigStore(), Path("/dev/null")
+            )
+            preview_rows = await preview_adapter.preview_for_settings(settings_obj)
+        except Exception as exc:
+            preview_error = f"Preview unavailable: {exc}"
+
     csrf_token = request.state.csrf_token
     response = templates.TemplateResponse(
         request=request,
@@ -1466,6 +1501,8 @@ async def adapters_edit_form(
             "tile_attribution": tile_attribution,
             "api_key_missing": api_key_missing,
             "requires_api_key_alias": requires_api_key_alias,
+            "preview_rows": preview_rows,
+            "preview_error": preview_error,
         },
     )
     return response