feat: Add initial backend server with user authentication, session management, and playlist CRUD functionality.

bin/middleware/auth_middleware.dart

@@ -0,0 +1,57 @@
+import 'package:shelf/shelf.dart';
+import '../database/database.dart';
+
+/// Middleware to authenticate requests
+Middleware authMiddleware(AppDatabase db) {
+  return (Handler handler) {
+    return (Request request) async {
+      // Skip auth for login endpoint
+      if (request.url.path.startsWith('api/auth/login')) {
+        return handler(request);
+      }
+
+      // Extract token
+      final token = _extractToken(request);
+      if (token == null) {
+        return Response(401, body: 'Unauthorized');
+      }
+
+      // Verify session
+      final session = db.findSessionByToken(token);
+      if (session == null) {
+        return Response(401, body: 'Invalid or expired session');
+      }
+
+      // Add userId to context
+      final updatedRequest = request.change(context: {
+        ...request.context,
+        'userId': session.userId,
+      });
+
+      return handler(updatedRequest);
+    };
+  };
+}
+
+/// Extract token from Authorization header or cookie
+String? _extractToken(Request request) {
+  // Try Authorization header first
+  final authHeader = request.headers['authorization'];
+  if (authHeader != null && authHeader.startsWith('Bearer ')) {
+    return authHeader.substring(7);
+  }
+
+  // Try cookie
+  final cookie = request.headers['cookie'];
+  if (cookie != null) {
+    final parts = cookie.split(';');
+    for (final part in parts) {
+      final trimmed = part.trim();
+      if (trimmed.startsWith('session=')) {
+        return trimmed.substring(8);
+      }
+    }
+  }
+
+  return null;
+}