Notifications, and automatic cleanup of empty folders

Settings → Notifications sends a message to Discord, Pushover, ntfy,
Gotify or a JSON webhook when a scan finds songs that newly need
review, when auto-link replaced identical files, and when an automatic
cleanup ran. Each event can be turned off and each service tested; an
optional Playlink address adds a link back to the right page.

A new per-pair option removes empty slave folders after each scan. Only
folders with no file at all inside are removed (into the quarantine,
recorded in History); folders holding any file wait in Cleanup, folders
changed within the settle time are skipped, and dry run removes nothing.

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: opt-in automatic removal of empty slave folders after each scan.
+	`ALTER TABLE pairs ADD COLUMN auto_cleanup_empty INTEGER NOT NULL DEFAULT 0;`,
 }
 
 // SchemaVersion is the newest database schema this build knows.