feat: pre-zip download all and photo replacement by name (#312, #313)

Pre-zip downloads:
- Generate ZIP in background after photo mutations (upload/delete/watermark change)
- Serve cached zip with Content-Length for instant downloads and native progress bar
- Falls back to on-the-fly streaming when no cache exists yet
- Frontend uses browser-native download when zip is ready (no blob buffering)
- New downloadZipService with debounced regeneration and in-memory locking

Photo replacement:
- Admin upload form gets "Replace existing photos with same name" checkbox
- Matches by original_filename (case-insensitive) within the same event
- Preserves photo ID, position, feedback, category, and visibility
- Updates file, thumbnail, dimensions, EXIF capture date on replacement
- Ambiguous matches (multiple photos with same name) skip replacement with warning
- New photoReplacementService with findReplacementCandidate and replacePhoto

frontend/src/services/gallery.service.ts

@@ -82,12 +82,26 @@ export const galleryService = {
   },
 
   // Download all photos as ZIP
-  async downloadAllPhotos(slug: string): Promise<void> {
+  // When a pre-generated zip is available, use native browser download (Content-Length → progress bar).
+  // Otherwise fall back to blob download.
+  async downloadAllPhotos(slug: string, zipReady?: boolean): Promise<void> {
+    if (zipReady) {
+      // Native browser download — the server sends Content-Length so
+      // the browser shows a real progress bar and mobile doesn't crash.
+      const link = document.createElement('a');
+      link.href = `/api/gallery/${slug}/download-all`;
+      link.setAttribute('download', `${slug}.zip`);
+      document.body.appendChild(link);
+      link.click();
+      link.remove();
+      return;
+    }
+
+    // Fallback: blob download (no Content-Length, buffered in memory)
     const response = await api.get(`/gallery/${slug}/download-all`, {
       responseType: 'blob',
     });
 
-    // Create download link
     const url = window.URL.createObjectURL(new Blob([response.data]));
     const link = document.createElement('a');
     link.href = url;