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/hooks/useGallery.ts

@@ -67,7 +67,8 @@ export const useDownloadPhoto = () => {
 
 export const useDownloadAllPhotos = () => {
   return useMutation({
-    mutationFn: (slug: string) => galleryService.downloadAllPhotos(slug),
+    mutationFn: ({ slug, zipReady }: { slug: string; zipReady?: boolean }) =>
+      galleryService.downloadAllPhotos(slug, zipReady),
     onSuccess: () => {
       toast.success('Download started');
     },