Migrate the ingester queue storage from raw aiosqlite to SQLAlchemy Core
async. The backend is chosen by ingester.queue.dburi: a SQLAlchemy async
URL points the queue at a database server, and SQLite remains the default
when unset. The Postgres path claims jobs with FOR UPDATE SKIP LOCKED so
multiple ingester processes can share one queue; SQLite caps the pool to a
single connection to keep the select-then-update claim atomic.
