Spike: SchemaWatcher keeps the _internal catalog current off the request path

Per-database schema_watch modes (owned / external / immutable):
- write path reads PRAGMA schema_version after every write task and
  rebuilds that one database's catalog before the write returns
- external files are polled with a stat() fingerprint prefilter
  (racily-clean fingerprints re-verified), PRAGMA schema_version only on
  change, rebuild only when the version moved or the inode changed
- immutable databases are scanned once
- remove_database deletes its catalog rows explicitly; startup deletes
  rows for databases that are not attached (no per-refresh full scans)
- catalog reads use one short-lived connection per database inside one
  read transaction; shared-cache memory databases go through their
  write thread
- replaced/deleted files invalidate cached connections (generation)
- fingerprints persisted in catalog_databases.fingerprint (migration
  0002) so restarts skip unchanged files
- new setting schema_watch_interval_ms (default 1000, 0 = no polling)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwkLo2rJWidpkDVT3HkvNv

tests/test_api.py

@@ -767,6 +767,7 @@ async def test_settings_json(ds_client):
         "template_debug": False,
         "trace_debug": False,
         "base_url": "/",
+        "schema_watch_interval_ms": 1000,
     }