feat: photo rename and viewer delete with confirmation

Backend:
- add PATCH /api/photos/:id to rename original_filename (uploader/owner/admin)
- emit photo.updated SSE event

Frontend:
- inline filename editor in the right panel
- viewer: back button (arrow_back) on the left, delete on the right
- delete confirmed via a modal dialog; filmstrip updates after deletion

backend/internal/app/app.go

@@ -133,6 +133,7 @@ func (s *Server) Router() *gin.Engine {
 	authed.GET("/albums/:id/photos", photosHandler.List)
 	authed.POST("/albums/:id/photos", photosHandler.Upload)
 	authed.GET("/photos/:id", photosHandler.Get)
+	authed.PATCH("/photos/:id", photosHandler.Rename)
 	authed.DELETE("/photos/:id", photosHandler.Delete)
 	authed.GET("/photos/:id/download", photosHandler.Download)