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/api/server.go

@@ -105,6 +105,9 @@ func (s *Server) Handler() http.Handler {
 				r.Get("/empty-folders", s.handleEmptyFolders)
 				r.Post("/empty-folders/scan", s.handleEmptyFolderScan)
 				r.Post("/empty-folders/remove", s.handleRemoveEmptyFolders)
+				r.Get("/duplicates", s.handleDupes)
+				r.Post("/duplicates/scan", s.handleDupeScan)
+				r.Post("/duplicates/remove", s.handleRemoveDupes)
 				r.Post("/build", s.handleBuild)
 				r.Post("/remove", s.handleRemove)
 			})
@@ -797,6 +800,38 @@ var audioTypes = map[string]string{
 }
 
 // handleListen streams a song with range support, so the browser can seek.
+func (s *Server) handleDupes(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	writeJSON(w, 200, map[string]any{"scan": s.eng.DupeResult(id), "dry_run": s.st.DryRun(r.Context())})
+}
+func (s *Server) handleDupeScan(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	j, err := s.eng.StartDupeScan(id)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	writeJSON(w, 202, j)
+}
+func (s *Server) handleRemoveDupes(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body struct {
+		Files []string `json:"files"` // must come from the latest search
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if !s.preAction(w, r, id, "remove duplicate songs") {
+		return
+	}
+	j, err := s.eng.StartRemoveDupes(id, body.Files)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	s.log.Info("removing duplicate songs", "pair", id, "files", len(body.Files))
+	writeJSON(w, 202, j)
+}
 func (s *Server) handleListen(w http.ResponseWriter, r *http.Request) {
 	id, _ := idParam(r)
 	q := r.URL.Query()