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_internal_db.py

@@ -29,7 +29,8 @@ async def test_internal_migrations_recorded(ds_client):
         order by id
         """)
     assert [tuple(row) for row in migrations.rows] == [
-        ("datasette_internal", "0001_initial")
+        ("datasette_internal", "0001_initial"),
+        ("datasette_internal", "0002_catalog_fingerprint"),
     ]
 
 
@@ -61,7 +62,8 @@ async def test_internal_migrations_adopt_existing_internal_db(tmp_path):
         order by id
         """)
     assert [tuple(row) for row in migrations.rows] == [
-        ("datasette_internal", "0001_initial")
+        ("datasette_internal", "0001_initial"),
+        ("datasette_internal", "0002_catalog_fingerprint"),
     ]
 
     ds.close()