feat(db/m7-art-providers): migration 0020 + provider-hash queries

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.

internal/db/queries/coverart_settings.sql

@@ -44,3 +44,19 @@ UPDATE cover_art_sources_meta
    SET current_version = current_version + 1
  WHERE id = true
 RETURNING current_version;
+
+-- name: GetCoverArtProvidersHash :one
+SELECT last_registered_providers_hash
+  FROM cover_art_sources_meta
+ WHERE id = true;
+
+-- name: BumpVersionAndSetProvidersHash :one
+-- Atomically increments the version and stores the new provider-set
+-- hash. Called at boot when the registered-provider set has changed
+-- since the last start, so 'none' rows become eligible for retry.
+-- RETURNING gives the new version without a follow-up SELECT.
+UPDATE cover_art_sources_meta
+   SET current_version = current_version + 1,
+       last_registered_providers_hash = $1
+ WHERE id = true
+RETURNING current_version;