feat: increase file upload limit from 20 to 500 with performance optimizations

Backend changes:
- Update multer configuration to accept up to 500 files per upload
- Implement batch processing (10 files per transaction) for better performance
- Add memory-efficient Sharp configuration for thumbnail generation
- Increase Express body parser limits to handle large payloads
- Add proper error handling and reporting for partial upload failures

Frontend changes:
- Update validation to allow 500 files maximum
- Implement chunked uploads (50 files per chunk) to prevent timeouts
- Add progress tracking with chunk information display
- Update error messages and translations (EN/DE)

Performance optimizations:
- Disable Sharp cache to prevent memory buildup
- Limit Sharp concurrency to 2 operations
- Use sequential read for large images
- Process files in database transaction batches
- Return detailed upload results including success/failure counts

This implementation ensures the application can handle large photo uploads
efficiently without running into memory or timeout issues.

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

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

frontend/src/i18n/locales/de.json

@@ -47,7 +47,10 @@
     "uploadComplete": "Upload abgeschlossen!",
     "uploadFailed": "Upload fehlgeschlagen",
     "someFilesFailed": "Einige Dateien konnten nicht hochgeladen werden",
-    "uploadPhotos": "Fotos hochladen"
+    "uploadPhotos": "Fotos hochladen",
+    "maxFilesReached": "Maximal 500 Dateien erlaubt",
+    "someFilesSkipped": "Einige Dateien wurden übersprungen (500 Dateien Limit)",
+    "tooManyFiles": "Maximal 500 Dateien können gleichzeitig hochgeladen werden"
   },
   "navigation": {
     "dashboard": "Dashboard",

frontend/src/i18n/locales/en.json

@@ -48,9 +48,9 @@
     "uploadFailed": "Upload failed",
     "someFilesFailed": "Some files failed to upload",
     "uploadPhotos": "Upload Photos",
-    "maxFilesReached": "Maximum 20 files allowed",
-    "someFilesSkipped": "Some files were skipped (20 file limit)",
-    "tooManyFiles": "Maximum 20 files can be uploaded at once"
+    "maxFilesReached": "Maximum 500 files allowed",
+    "someFilesSkipped": "Some files were skipped (500 file limit)",
+    "tooManyFiles": "Maximum 500 files can be uploaded at once"
   },
   "navigation": {
     "dashboard": "Dashboard",