feat: show original filename in admin UI (#184)

Surface the existing original_filename from the database in the admin
photo grid hover overlay and photo viewer sidebar, so photographers can
correlate uploaded images with their Lightroom/disk originals. Only shown
when it differs from the system-generated filename. Gallery guests remain
unaffected.

frontend/src/services/photos.service.ts

@@ -3,6 +3,7 @@ import { api } from '../config/api';
 export interface AdminPhoto {
   id: number;
   filename: string;
+  original_filename?: string;
   path: string;
   url: string;
   thumbnail_url: string | null;