Add finding and removing folders without music

The new Cleanup tab lists the top-most folders in either library that
contain no music file (per the pair's extensions). Removing them needs
confirmation, plus an explicit acknowledgement when they still hold
files; each folder is re-checked for music and moved into the
quarantine (undoable from History, purged after the retention period).

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

internal/api/server.go

@@ -101,6 +101,9 @@ func (s *Server) Handler() http.Handler {
 				r.Post("/renames", s.handleRenames)
 				r.Post("/link-identical", s.handleLinkIdentical)
 				r.Get("/tree", s.handleTree)
+				r.Get("/empty-folders", s.handleEmptyFolders)
+				r.Post("/empty-folders/scan", s.handleEmptyFolderScan)
+				r.Post("/empty-folders/remove", s.handleRemoveEmptyFolders)
 				r.Post("/build", s.handleBuild)
 				r.Post("/remove", s.handleRemove)
 			})
@@ -740,3 +743,46 @@ func (s *Server) preAction(w http.ResponseWriter, r *http.Request, pairID int64,
 	}
 	return true
 }
+
+// ---- folders without music ----
+
+func (s *Server) handleEmptyFolders(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	writeJSON(w, 200, map[string]any{"scan": s.eng.NoMusicResult(id), "dry_run": s.st.DryRun(r.Context())})
+}
+
+func (s *Server) handleEmptyFolderScan(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body struct {
+		Side string `json:"side"`
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	j, err := s.eng.StartNoMusicScan(id, body.Side)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	writeJSON(w, 202, j)
+}
+
+func (s *Server) handleRemoveEmptyFolders(w http.ResponseWriter, r *http.Request) {
+	id, _ := idParam(r)
+	var body struct {
+		Folders []string `json:"folders"` // must come from the latest scan
+	}
+	if !readJSON(w, r, &body) {
+		return
+	}
+	if !s.preAction(w, r, id, "remove folders without music") {
+		return
+	}
+	j, err := s.eng.StartRemoveFolders(id, body.Folders)
+	if err != nil {
+		opErr(w, err)
+		return
+	}
+	s.log.Info("removing folders without music", "pair", id, "folders", len(body.Folders))
+	writeJSON(w, 202, j)
+}