feat(library): fold a missing track into its on-disk replacement (M485 #5286 #5287)

A track marked missing whose replacement is already on disk under another
row — on the operator's library 355 of 451 missing tracks, nearly all Lidarr
mp3 -> flac upgrades — is folded into the replacement: likes, plays, playlist
entries and tags move across and the missing row goes. Move adoption could
not catch these: the replacements were re-encodes (no shared audio hash) with
no recording MBID at import.

Pairs (ListMissingTrackPairs): the same recording MBID within the album
group, or the same album row and title ignoring case. Each side must have
exactly one candidate; conflicting MBIDs refuse a pair. No duration or track
position gate: on the 142 pairs known to be one recording, 18% differed by
over 2s and the poorly tagged set is where numbering is broken (spike #5274).

Each pair folds in its own transaction after locking both rows and checking
the pair still holds. Runs automatically (operator, 2026-10-07) after a full
scan, after a watcher batch that added or updated tracks, and after an
AcoustID pass that matched any track. The first scan after deploy repairs the
existing rows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/acoustid_lookup.go

@@ -215,6 +215,14 @@ func (w *AcoustIDLookupWorker) runOnce(ctx context.Context) {
 			"processed", res.Processed, "matched", res.Matched, "ambiguous", res.Ambiguous,
 			"no_match", res.NoMatch, "failed", res.Failed, "inconclusive", res.Inconclusive)
 	}
+	// A recording MBID AcoustID just gave a replacement may be the one its
+	// missing predecessor carried (M485): most of the operator's stale rows
+	// became pairable exactly this way.
+	if res.Matched > 0 && ctx.Err() == nil {
+		if _, err := PairMissingTracks(ctx, w.pool, w.logger); err != nil && ctx.Err() == nil {
+			w.logger.Warn("acoustid lookup: missing-pair pass failed", "err", err)
+		}
+	}
 }
 
 func (w *AcoustIDLookupWorker) setStatus(f func(*AcoustIDLookupStatus)) {