fix(tests): replace stub tests with real DB migration tests

- Replace pytest.skip stubs with actual DB tests against central_test
- Test backfill for all three adapters (nws, firms, usgs_quake)
- Test FK RESTRICT, NOT NULL, and FK validation constraints
- Test schema changes (source dropped, adapter exists with constraints)
- Delete stale sql/schema.sql (migrations are sole source of truth)
- Update docs/migrations.md with schema.sql removal note

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

docs/migrations.md

@@ -1,5 +1,11 @@
 # Migration policy
 
+## Migrations are the sole source of truth
+
+The `sql/migrations/` directory contains all schema definitions. There is
+no separate schema.sql file; use `pg_dump -s central` to generate a
+human-readable snapshot of the current schema when needed.
+
 ## Migrations must be idempotent
 
 New migration files (007+) must use guards so they can be safely