fix: photo upload issues with file limit and date formatting

- Add 20-file limit validation to PhotoUpload component
- Prevent Multer "Unexpected field" errors by enforcing client-side limit
- Fix JSON parsing error in dateFormatter when value is already an object
- Add missing translation keys for upload error messages
- Handle both string and object values for date format settings

These fixes resolve the 400 error when uploading more than 20 files
and the "Unexpected token o in JSON" error during email queue creation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/admin/PhotoUpload.tsx

@@ -32,6 +32,20 @@ export const PhotoUpload: React.FC<PhotoUploadProps> = ({ eventId, onUploadCompl
     const imageFiles = files.filter(file => 
       ['image/jpeg', 'image/png', 'image/webp'].includes(file.type)
     );
+    
+    // Check total file count with existing files
+    const totalFiles = selectedFiles.length + imageFiles.length;
+    if (totalFiles > 20) {
+      const allowedNewFiles = 20 - selectedFiles.length;
+      if (allowedNewFiles <= 0) {
+        toast.error(t('upload.maxFilesReached') || 'Maximum 20 files allowed');
+        return;
+      }
+      toast.warning(t('upload.someFilesSkipped') || `Only ${allowedNewFiles} more files can be added (20 max)`);
+      setSelectedFiles(prev => [...prev, ...imageFiles.slice(0, allowedNewFiles)]);
+      return;
+    }
+    
     setSelectedFiles(prev => [...prev, ...imageFiles]);
   };
 
@@ -41,6 +55,12 @@ export const PhotoUpload: React.FC<PhotoUploadProps> = ({ eventId, onUploadCompl
 
   const handleUpload = async () => {
     if (selectedFiles.length === 0) return;
+    
+    // Validate file count
+    if (selectedFiles.length > 20) {
+      toast.error(t('upload.tooManyFiles') || 'Maximum 20 files can be uploaded at once');
+      return;
+    }
 
     setIsUploading(true);
     setUploadProgress(0);