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>

sql/schema.sql

@@ -1,9 +1,12 @@
 -- Central Data Hub schema
 -- PostgreSQL 16 + TimescaleDB + PostGIS
+-- NOTE: Migrations in sql/migrations/ are the source of truth.
+-- This file is for reference and initial setup only.
 
 CREATE TABLE IF NOT EXISTS events (
     id              TEXT NOT NULL,                     -- CloudEvent id
-    source          TEXT NOT NULL,                     -- adapter identity
+    adapter         TEXT NOT NULL                      -- adapter identity (FK to config.adapters.name)
+                    REFERENCES config.adapters(name) ON DELETE RESTRICT,
     category        TEXT NOT NULL,                     -- "wx.alert.<type>"
     time            TIMESTAMPTZ NOT NULL,              -- event-time UTC
     expires         TIMESTAMPTZ,
@@ -21,6 +24,9 @@ SELECT create_hypertable('events', 'time', if_not_exists => TRUE);
 CREATE INDEX IF NOT EXISTS events_category_time_idx
     ON events (category, time DESC);
 
+CREATE INDEX IF NOT EXISTS events_adapter_received_idx
+    ON events (adapter, received DESC);
+
 CREATE INDEX IF NOT EXISTS events_geom_gist
     ON events USING GIST (geom);