docs(db/m7-coverage-gauge): note CHECK-constraint sync + bucket invariant

Code-review polish on GetAlbumCoverageRollup's comment block:
- Records the invariant that with_art + pending + settled = total
  (and that pending_no_mbid is a subset of pending, not a fourth
  bucket).
- Notes that the IN ('sidecar','embedded','mbcaa') list must stay
  in sync with the cover_art_source CHECK constraint in migration
  0016 — without this note a future source addition could silently
  undercount with_art.

internal/db/dbq/albums.sql.go

@@ -145,6 +145,14 @@ type GetAlbumCoverageRollupRow struct {
 // of pending; the UI surfaces it separately so operators can see how
 // many "pending" albums are blocked on missing MBID and won't be
 // helped by another scan.
+//
+// Invariant: with_art + pending + settled = total. (pending_no_mbid is
+// not part of the sum — it's a subset of pending, surfaced separately.)
+//
+// The IN list below ('sidecar','embedded','mbcaa') must stay in sync
+// with the cover_art_source CHECK constraint in migration
+// 0016_album_cover_source.up.sql. If a new source value is added there
+// without updating this query, with_art will silently undercount.
 func (q *Queries) GetAlbumCoverageRollup(ctx context.Context) (GetAlbumCoverageRollupRow, error) {
 	row := q.db.QueryRow(ctx, getAlbumCoverageRollup)
 	var i GetAlbumCoverageRollupRow

internal/db/queries/albums.sql

@@ -132,6 +132,14 @@ SELECT a.id          AS album_id,
 -- of pending; the UI surfaces it separately so operators can see how
 -- many "pending" albums are blocked on missing MBID and won't be
 -- helped by another scan.
+--
+-- Invariant: with_art + pending + settled = total. (pending_no_mbid is
+-- not part of the sum — it's a subset of pending, surfaced separately.)
+--
+-- The IN list below ('sidecar','embedded','mbcaa') must stay in sync
+-- with the cover_art_source CHECK constraint in migration
+-- 0016_album_cover_source.up.sql. If a new source value is added there
+-- without updating this query, with_art will silently undercount.
 SELECT
   COUNT(*)                                                                   AS total,
   COUNT(*) FILTER (WHERE cover_art_source IN ('sidecar','embedded','mbcaa')) AS with_art,