feat(schema): add adapter column to events, drop source

Replaces module-path-based source column (e.g. "central/adapters/nws")
with stable adapter identifier (e.g. "nws") that foreign-keys to
config.adapters.name.

Migration 011:
- ADD COLUMN adapter TEXT
- Backfill via REPLACE(source, 'central/adapters/', '')
- SET NOT NULL + FK RESTRICT
- CREATE INDEX (adapter, received DESC) for dashboard queries
- DROP COLUMN source

Code changes:
- Event model: source field renamed to adapter
- All adapters: use adapter="name" instead of source="central/adapters/name"
- Archive: write adapter column instead of source

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_models.py

@@ -25,7 +25,7 @@ def sample_event(sample_geo: Geo) -> Event:
     """Sample Event object for testing."""
     return Event(
         id="urn:central:nws:alert:KBOI-202401151200-SVR",
-        source="central/adapters/nws",
+        adapter="nws",
         category="wx.alert.severe_thunderstorm_warning",
         time=datetime(2024, 1, 15, 12, 0, 0, tzinfo=timezone.utc),
         expires=datetime(2024, 1, 15, 13, 0, 0, tzinfo=timezone.utc),
@@ -75,7 +75,7 @@ class TestSubjectForEvent:
         )
         event = Event(
             id="test-zone",
-            source="test",
+            adapter="nws",
             category="wx.alert.winter_storm_warning",
             time=datetime(2024, 1, 15, 12, 0, 0, tzinfo=timezone.utc),
             geo=geo,
@@ -89,7 +89,7 @@ class TestSubjectForEvent:
         geo = Geo(regions=[], primary_region=None)
         event = Event(
             id="test-unknown",
-            source="test",
+            adapter="nws",
             category="wx.alert.test",
             time=datetime(2024, 1, 15, 12, 0, 0, tzinfo=timezone.utc),
             geo=geo,
@@ -144,7 +144,7 @@ class TestCloudEventsWire:
         """When severity is None, centralseverity is omitted entirely."""
         event = Event(
             id="test-no-severity",
-            source="test",
+            adapter="nws",
             category="wx.alert.test",
             time=datetime(2024, 1, 15, 12, 0, 0, tzinfo=timezone.utc),
             severity=None,  # Explicitly None