Relaxes the artists/albums art_source CHECK constraints to allow the
new 'deezer' and 'lastfm' source values alongside the existing
accepted set. Adds cover_art_sources_meta.last_registered_providers_hash
for the boot-time auto-bump detection: when the registered-provider
set changes between deploys, we bump current_version once so 'none'
rows become eligible for retry against the new chain.
Two sqlc queries added: GetCoverArtProvidersHash reads the stored
hash; BumpVersionAndSetProvidersHash atomically increments the
version and stores a new hash, returning the new version. Both used
by the boot logic in a later task.
