feat(server/m7-380): one-shot MBID backfill worker for existing libraries

Adds a boot-time goroutine that walks albums with NULL mbid, re-reads
tags from one track per album via dhowden/tag, and persists album + artist
MBIDs. Healed albums also get their cover_art_source='none' cleared so the
enricher's next batch retries the MBCAA fetch. Caps at 5000 albums per
boot to avoid stalling startup on huge libraries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/queries/covers.sql

@@ -44,6 +44,14 @@ UPDATE albums
        cover_art_source = NULL
  WHERE id = $1;
 
+-- name: ClearAlbumCoverNone :exec
+-- M7 #380: when MBID backfill heals an album, clear its 'none' cover_art_source
+-- back to NULL so the enricher's next batch retries the MBCAA fetch.
+-- Idempotent — only updates rows currently stamped 'none'.
+UPDATE albums
+   SET cover_art_source = NULL
+ WHERE id = $1 AND cover_art_source = 'none';
+
 -- name: CountAlbumCoverSources :one
 -- Diagnostic for the admin overview: counts by source, including NULL.
 -- Returns one row with named tallies.