feat: add city avatar upload functionality and enhance city management features

backend/src/middleware/cityUpload.js

@@ -0,0 +1,30 @@
+const multer = require('multer');
+const path = require('path');
+const fs = require('fs');
+
+const uploadDir = path.join(__dirname, '../../public/uploads/cities');
+if (!fs.existsSync(uploadDir)) fs.mkdirSync(uploadDir, { recursive: true });
+
+const allowedExtensions = new Set(['.jpg', '.jpeg', '.png', '.webp']);
+const allowedMimeTypes = new Set(['image/jpeg', 'image/png', 'image/webp']);
+
+const storage = multer.diskStorage({
+  destination: (_req, _file, callback) => callback(null, uploadDir),
+  filename: (req, file, callback) => {
+    const cityId = Number(req.params.cityId);
+    const extension = path.extname(file.originalname).toLowerCase();
+    callback(null, `city-${cityId}-${Date.now()}${extension}`);
+  }
+});
+
+function fileFilter(_req, file, callback) {
+  const extension = path.extname(file.originalname).toLowerCase();
+  if (allowedExtensions.has(extension) && allowedMimeTypes.has(file.mimetype)) return callback(null, true);
+  return callback(new Error('Only JPEG, PNG, and WEBP images are allowed'));
+}
+
+module.exports = multer({
+  storage,
+  limits: { fileSize: 5 * 1024 * 1024 },
+  fileFilter
+});