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>

src/central/models.py

@@ -23,7 +23,7 @@ class Event(BaseModel):
     model_config = ConfigDict(extra="forbid", frozen=True)
 
     id: str  # unique, stable across republish
-    source: str  # adapter identity, e.g. "central/adapters/nws"
+    adapter: str  # adapter identity, e.g. "nws"
     category: str  # e.g. "wx.alert.severe_thunderstorm_warning" or "fire.hotspot.viirs_snpp.high"
     time: datetime  # event-time UTC, not processing-time
     expires: datetime | None = None