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>

backend/server.js

@@ -125,9 +125,9 @@ const authLimiter = rateLimit({
 app.use('/api/', limiter);
 app.use('/api/auth', authLimiter);
 
-// Body parsing middleware
-app.use(express.json());
-app.use(express.urlencoded({ extended: true }));
+// Body parsing middleware with increased limits for large uploads
+app.use(express.json({ limit: '100mb' }));
+app.use(express.urlencoded({ extended: true, limit: '100mb' }));
 
 // Maintenance mode middleware - add after body parsing but before routes
 app.use(maintenanceMiddleware);