Find and remove duplicate songs within the slave library

A new Duplicates filter in Review finds songs filed more than once in the
slave library, such as a single next to its album version, and recommends
which copy to keep. Removed copies go to the quarantine and can be restored.

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

internal/engine/engine.go

@@ -46,6 +46,9 @@ type Engine struct {
 	emptyMu sync.Mutex
 	empty   map[int64]*NoMusicScan // latest scan for folders without music, per pair
 
+	dupeMu sync.Mutex
+	dupes  map[int64]*DupeScan // latest duplicate search, per pair
+
 	jobsMu sync.Mutex
 	jobs   map[int64]*Job
 	nextID int64