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/migrations/0020_relax_art_source_checks.down.sql

@@ -0,0 +1,18 @@
+-- Restore the prior tighter CHECK constraints (pre-Deezer/Last.fm).
+-- This will fail if any rows currently have artist_art_source =
+-- 'deezer' or 'lastfm' (likewise album); operator must clear those
+-- rows manually before rolling back.
+
+ALTER TABLE artists DROP CONSTRAINT IF EXISTS artists_artist_art_source_check;
+ALTER TABLE artists
+    ADD CONSTRAINT artists_artist_art_source_check
+    CHECK (artist_art_source IS NULL
+           OR artist_art_source IN ('theaudiodb','none'));
+
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_art_source_check;
+ALTER TABLE albums
+    ADD CONSTRAINT albums_cover_art_source_check
+    CHECK (cover_art_source IS NULL
+           OR cover_art_source IN ('embedded','sidecar','mbcaa','theaudiodb','none'));
+
+ALTER TABLE cover_art_sources_meta DROP COLUMN IF EXISTS last_registered_providers_hash;

internal/db/migrations/0020_relax_art_source_checks.up.sql

@@ -0,0 +1,22 @@
+-- Relax cover-art and artist-art source CHECK constraints so the new
+-- Deezer and Last.fm providers (next slice) can persist their IDs as
+-- accepted source values. Also add the
+-- last_registered_providers_hash column to cover_art_sources_meta for
+-- boot-time auto-bump detection (when the registered-provider set
+-- changes, we bump current_version once so 'none' rows get
+-- re-eligibled).
+
+ALTER TABLE artists DROP CONSTRAINT IF EXISTS artists_artist_art_source_check;
+ALTER TABLE artists
+    ADD CONSTRAINT artists_artist_art_source_check
+    CHECK (artist_art_source IS NULL
+           OR artist_art_source IN ('theaudiodb','deezer','lastfm','none'));
+
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_art_source_check;
+ALTER TABLE albums
+    ADD CONSTRAINT albums_cover_art_source_check
+    CHECK (cover_art_source IS NULL
+           OR cover_art_source IN ('embedded','sidecar','mbcaa','theaudiodb','deezer','lastfm','none'));
+
+ALTER TABLE cover_art_sources_meta
+    ADD COLUMN IF NOT EXISTS last_registered_providers_hash text NOT NULL DEFAULT '';