feat: Initial MVP with home server identity model

- User model with Ed25519 keypair identity
- Register/login on home server (creates keypair)
- Join guest servers with signature verification
- Public endpoints for cross-server identity
- Channel and message CRUD
- JWT authentication middleware

Architecture: Home server model, no federation, client-driven sync

lib/src/auth/auth_middleware.dart

@@ -0,0 +1,45 @@
+import 'package:relic/relic.dart';
+
+import '../../config/config.dart';
+import '../utils/extensions.dart';
+import 'crypto_service.dart';
+
+final userIdProperty = ContextProperty<String>('userId');
+final usernameProperty = ContextProperty<String>('username');
+final userLocalIdProperty = ContextProperty<int>('userLocalId');
+
+/// JWT authentication middleware
+final jwtMiddleware = createMiddleware(
+  onRequest: (request) {
+    final authHeader = request.headers.authorization;
+
+    if (authHeader is! BearerAuthorizationHeader) {
+      return Response.unauthorized(
+        body: Body.fromString('Bearer token required'),
+      );
+    }
+
+    final payload = AuthService.verifyToken(authHeader.token);
+    if (payload == null) {
+      return Response.forbidden(
+        body: Body.fromString('Invalid or expired token'),
+      );
+    }
+
+    final userId = payload['sub'] as String?;
+    final username = payload['username'] as String?;
+
+    if (userId == null) {
+      return Response.forbidden(
+        body: Body.fromString('Invalid token payload'),
+      );
+    }
+
+    userIdProperty[request] = userId;
+    if (username != null) {
+      usernameProperty[request] = username;
+    }
+
+    return null; // Continue to handler
+  },
+);