Schema watcher: never recreate deleted files; O(1) derived-table cache key

- write connections reopened after the watcher saw the file replaced or
  deleted use mode=rw, so a write to a deleted database fails instead of
  silently creating a new empty file
- derived_table_dependencies() keys its cache on the schema_version the
  watcher stored in the catalog (plus its scan count) instead of running
  PRAGMA schema_version on a pooled read connection - allowed_resources()
  calls it for every database on every page

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

tests/test_schema_watcher.py

@@ -440,6 +440,10 @@ async def test_external_file_deleted_and_recreated(tmp_path, db_path):
     await wait_for(gone)
     response = await ds.client.get("/ext/t.json")
     assert response.status_code in (404, 500)
+    # Writes fail instead of silently creating a new empty file
+    with pytest.raises(sqlite3.OperationalError):
+        await ds.get_database("ext").execute_write("insert into t (a) values ('x')")
+    assert not os.path.exists(db_path)
     make_db(db_path, sql="create table t (id integer primary key, a text, back text)")
 
     async def back():