Fix upload directory path issue

- Change UPLOAD_DIR from ../uploads to ./uploads in server/.env
- Use absolute path with process.cwd() in upload middleware
- Ensures uploads directory is created in correct location

.kiro/specs/beach-litter-mapping/tasks.md

@@ -145,7 +145,7 @@
 
 ## Cleanup Tracking System
 
-- [-] 18. Implement "Mark as Cleaned" functionality
+- [x] 18. Implement "Mark as Cleaned" functionality
   - Add `cleaned_at` timestamp field to reports table
   - Add `cleaned_by_user_id` field to track who marked it as cleaned
   - Create POST /api/reports/:id/mark-cleaned backend endpoint

server/src/middleware/upload.ts

@@ -2,8 +2,8 @@ import multer from 'multer';
 import path from 'path';
 import fs from 'fs';
 
-// Ensure upload directory exists
-const uploadDir = process.env.UPLOAD_DIR || './uploads';
+// Ensure upload directory exists - use absolute path
+const uploadDir = process.env.UPLOAD_DIR || path.join(process.cwd(), 'uploads');
 if (!fs.existsSync(uploadDir)) {
   fs.mkdirSync(uploadDir, { recursive: true });
 }