perf(database): index enclosure URL digest as bytea instead of hex

The enclosures_user_entry_url_unique_idx expression index stored the URL
digest as encode(sha256(url::bytea), 'hex'), a 64-byte hex text encoding
of a 32-byte value. Index the raw sha256(url::bytea) bytea instead to
roughly halve the index on disk, and update the matching ON CONFLICT
clause in createEnclosure so its inference still resolves to the index.

On 150k enclosures the index shrank from 27 MB to 11 MB (~59%).