feat(photos): fix trash permanent delete and add empty trash

Permanently deleting a photo whose album was deleted failed because
authorization resolved through the album. Fall back to uploader
ownership when the photo is albumless. Add a bulk DELETE /trash that
removes every trashed photo the caller may delete (uncapped listing)
and returns the deleted count, with an «Очистить корзину» action on the
trash screen.

backend/internal/photos/handlers.go

@@ -166,6 +166,15 @@ func (h *Handler) TrashCount(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"count": n})
 }
 
+func (h *Handler) EmptyTrash(c *gin.Context) {
+	n, err := h.svc.EmptyTrash(c.Request.Context(), auth.UserID(c))
+	if err != nil {
+		httpx.RespondError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"deleted": n})
+}
+
 func (h *Handler) Tags(c *gin.Context) {
 	tags, err := h.svc.Tags(c.Request.Context(), auth.UserID(c), c.Query("q"))
 	if err != nil {