Flag slave-only songs whose album or artist the master has

Scans now mark slave-only songs as missing from the master when the master
has their album folder or at least their artist folder. A new Missing from
master filter in Review lists them, split by album and artist.

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

internal/store/store.go

@@ -140,6 +140,9 @@ var migrations = []string{
 	// 10: library type (music or soundtracks) and the step-by-step fixer's progress.
 	`ALTER TABLE pairs ADD COLUMN library_type TEXT NOT NULL DEFAULT 'music';
 	ALTER TABLE pairs ADD COLUMN fixer_state TEXT NOT NULL DEFAULT '';`,
+
+	// 11: slave-only songs whose album or artist the master has.
+	`ALTER TABLE matches ADD COLUMN missing TEXT NOT NULL DEFAULT '';`,
 }
 
 // SchemaVersion is the newest database schema this build knows.