fix: handle broken thumbnails and incomplete uploads

- Implement automatic thumbnail regeneration for corrupted files
- Add validation middleware to check uploaded files before processing
- Improve error handling for "Premature end of input file" errors
- Delete and regenerate thumbnails when serving fails
- Add file integrity checks during upload process
- Better error reporting for failed uploads

This fixes the VipsJpeg errors and ensures reliable thumbnail generation.

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

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

backend/src/middleware/uploadValidation.js

@@ -0,0 +1,90 @@
+const fs = require('fs').promises;
+const path = require('path');
+const sharp = require('sharp');
+const logger = require('../utils/logger');
+
+/**
+ * Validate uploaded file is complete and not corrupted
+ */
+async function validateUploadedFile(filePath) {
+  try {
+    // Check file exists and has size
+    const stats = await fs.stat(filePath);
+    if (stats.size === 0) {
+      throw new Error('File is empty');
+    }
+    
+    // For image files, verify they can be read by Sharp
+    const ext = path.extname(filePath).toLowerCase();
+    const imageExtensions = ['.jpg', '.jpeg', '.png', '.gif', '.webp'];
+    
+    if (imageExtensions.includes(ext)) {
+      // Try to read metadata - this will fail if image is corrupted
+      const metadata = await sharp(filePath).metadata();
+      
+      if (!metadata.width || !metadata.height) {
+        throw new Error('Invalid image dimensions');
+      }
+      
+      // Check for reasonable dimensions
+      if (metadata.width < 10 || metadata.height < 10) {
+        throw new Error('Image dimensions too small');
+      }
+      
+      return true;
+    }
+    
+    return true;
+  } catch (error) {
+    logger.error(`File validation failed for ${filePath}:`, error.message);
+    throw error;
+  }
+}
+
+/**
+ * Middleware to validate uploaded files after multer processing
+ */
+async function validateUploadedFiles(req, res, next) {
+  if (!req.files || req.files.length === 0) {
+    return next();
+  }
+  
+  const validFiles = [];
+  const invalidFiles = [];
+  
+  // Validate each file
+  for (const file of req.files) {
+    try {
+      await validateUploadedFile(file.path);
+      validFiles.push(file);
+    } catch (error) {
+      logger.warn(`Removing invalid upload ${file.originalname}: ${error.message}`);
+      invalidFiles.push({
+        filename: file.originalname,
+        error: error.message
+      });
+      
+      // Delete the invalid file
+      try {
+        await fs.unlink(file.path);
+      } catch (unlinkErr) {
+        logger.error(`Failed to delete invalid file ${file.path}:`, unlinkErr.message);
+      }
+    }
+  }
+  
+  // Update req.files to only include valid files
+  req.files = validFiles;
+  
+  // Store invalid files info for response
+  if (invalidFiles.length > 0) {
+    req.invalidFiles = invalidFiles;
+  }
+  
+  next();
+}
+
+module.exports = {
+  validateUploadedFile,
+  validateUploadedFiles
+};