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/src/services/imageProcessor.js

@@ -2,6 +2,10 @@ const sharp = require('sharp');
 const path = require('path');
 const fs = require('fs').promises;
 
+// Configure sharp for better memory management with large batches
+sharp.cache(false); // Disable cache to prevent memory buildup
+sharp.concurrency(2); // Limit concurrent operations
+
 const THUMBNAIL_WIDTH = 300;
 const getStoragePath = () => process.env.STORAGE_PATH || path.join(__dirname, '../../../storage');
 const getThumbnailPath = () => path.join(getStoragePath(), 'thumbnails');
@@ -15,16 +19,29 @@ async function generateThumbnail(imagePath) {
   // Ensure thumbnail directory exists
   await fs.mkdir(thumbnailDir, { recursive: true });
   
-  // Generate thumbnail
-  await sharp(imagePath)
-    .resize(THUMBNAIL_WIDTH, null, {
-      withoutEnlargement: true,
-      fit: 'inside'
+  try {
+    // Generate thumbnail with memory-efficient settings
+    await sharp(imagePath, { 
+      limitInputPixels: 268402689, // ~16k x 16k max
+      sequentialRead: true // More memory efficient for large images
     })
-    .jpeg({ quality: 80 })
-    .toFile(thumbnailPath);
-  
-  return path.relative(getStoragePath(), thumbnailPath);
+      .resize(THUMBNAIL_WIDTH, null, {
+        withoutEnlargement: true,
+        fit: 'inside'
+      })
+      .jpeg({ 
+        quality: 80,
+        progressive: true, // Progressive JPEG for better loading
+        mozjpeg: true // Better compression
+      })
+      .toFile(thumbnailPath);
+    
+    return path.relative(getStoragePath(), thumbnailPath);
+  } catch (error) {
+    console.error(`Failed to generate thumbnail for ${filename}:`, error);
+    // Return null if thumbnail generation fails, don't fail the whole upload
+    return null;
+  }
 }
 
 module.exports = { generateThumbnail };