feat(backend): implement comprehensive security and validation

Implement enterprise-grade security measures and input validation:

Security Features:
- Add Helmet.js for security headers (XSS, clickjacking, MIME protection)
- Implement rate limiting (5/15min for auth, 100/15min for API)
- Add Socket.IO JWT authentication middleware
- Fix JWT auth middleware (remove throw in catch, extend token to 7 days)
- Implement centralized error handling with AppError class
- Add CORS restrictive configuration

Input Validation:
- Add express-validator to all routes (auth, streets, tasks, posts, events, rewards, reports, users)
- Create comprehensive validation schemas in middleware/validators/
- Consistent error response format for validation failures

Additional Features:
- Add pagination middleware for all list endpoints
- Add Multer file upload middleware (5MB limit, image validation)
- Update .env.example with all required environment variables

Dependencies Added:
- helmet@8.1.0
- express-rate-limit@8.2.1
- express-validator@7.3.0
- multer@1.4.5-lts.1
- cloudinary@2.8.0

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

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

backend/middleware/validators/userValidator.js

@@ -0,0 +1,28 @@
+const { param, validationResult } = require("express-validator");
+
+const validate = (req, res, next) => {
+  const errors = validationResult(req);
+  if (!errors.isEmpty()) {
+    return res.status(400).json({
+      success: false,
+      errors: errors.array().map((err) => ({
+        field: err.path,
+        message: err.msg,
+      })),
+    });
+  }
+  next();
+};
+
+/**
+ * User ID validation
+ */
+const userIdValidation = [
+  param("id").isMongoId().withMessage("Invalid user ID"),
+  validate,
+];
+
+module.exports = {
+  userIdValidation,
+  validate,
+};