feat: refactor server with shelf_static, return JSON responses, add API docs

bin/middleware/auth_middleware.dart

@@ -1,6 +1,7 @@
 import 'package:shelf/shelf.dart';
 import 'package:dart_jsonwebtoken/dart_jsonwebtoken.dart';
 import 'package:dotenv/dotenv.dart';
+import 'dart:convert';
 
 class AuthMiddleware {
   final Future<Response> Function(Request, String) handler;
@@ -11,7 +12,7 @@ class AuthMiddleware {
     final authorization = request.headers['authorization'];
     
     if (authorization == null || !authorization.startsWith('Bearer ')) {
-      return Response(401, body: 'Authorization header missing or invalid');
+      return Response(401, body: jsonEncode({'error': 'Authorization header missing or invalid'}), headers: {'Content-Type': 'application/json'});
     }
     
     final token = authorization.substring(7);
@@ -25,9 +26,9 @@ class AuthMiddleware {
       
       return handler(request, login);
     } on JWTExpiredException {
-      return Response(401, body: 'Token expired');
+      return Response(401, body: jsonEncode({'error': 'Token expired'}), headers: {'Content-Type': 'application/json'});
     } on JWTException {
-      return Response(401, body: 'Invalid token');
+      return Response(401, body: jsonEncode({'error': 'Invalid token'}), headers: {'Content-Type': 'application/json'});
     }
   }
 }