feat(workspace): backlinks at the end of a note

GET /documents/{id}/backlinks lists the notes the user can open whose
blocks link [[Title]] (also |alias and #section, case-insensitive),
excluding the note itself and the trash. The collapsible "Verlinkt auf
diese Notiz" section shows each note with folder and the linking bullets;
a click jumps straight to the bullet.

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

internal/api/server.go

@@ -176,6 +176,7 @@ func (s *Server) Router() http.Handler {
 		r.Get("/api/v1/documents/trash", s.handleGetTrash)
 		r.Delete("/api/v1/documents/trash", s.handleEmptyTrash)
 		r.Post("/api/v1/documents/{id}/restore", s.handleRestoreDocument)
+		r.Get("/api/v1/documents/{id}/backlinks", s.handleGetBacklinks)
 		r.Delete("/api/v1/documents/{id}/permanent", s.handlePurgeDocument)
 		r.Post("/api/v1/documents/{id}/blocks/{blockId}/enhanced-task", s.handleCreateEnhancedTask)
 
@@ -1117,6 +1118,15 @@ func (s *Server) handleGetDocument(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, doc)
 }
 
+func (s *Server) handleGetBacklinks(w http.ResponseWriter, r *http.Request) {
+	links, err := s.store.GetBacklinks(auth.GetUserID(r.Context()), chi.URLParam(r, "id"))
+	if err != nil {
+		writeError(w, http.StatusNotFound, "Dokument nicht gefunden")
+		return
+	}
+	writeJSON(w, http.StatusOK, links)
+}
+
 func (s *Server) handleUpdateDocument(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
 	docID := chi.URLParam(r, "id")