feat(auth): authentication module with multi-tenant isolation (RFC-001)

Introduce an always-on auth layer with auto-created admin on first boot,
multi-tenant isolation for threads/stores, and a full setup/login flow.

Backend
- JWT access tokens with `ver` field for stale-token rejection; bump on
  password/email change
- Password hashing, HttpOnly+Secure cookies (Secure derived from request
  scheme at runtime)
- CSRF middleware covering both REST and LangGraph routes
- IP-based login rate limiting (5 attempts / 5-min lockout) with bounded
  dict growth and X-Forwarded-For bypass fix
- Multi-worker-safe admin auto-creation (single DB write, WAL once)
- needs_setup + token_version on User model; SQLite schema migration
- Thread/store isolation by owner; orphan thread migration on first admin
  registration
- thread_id validated as UUID to prevent log injection
- CLI tool to reset admin password
- Decorator-based authz module extracted from auth core

Frontend
- Login and setup pages with SSR guard for needs_setup flow
- Account settings page (change password / email)
- AuthProvider + route guards; skips redirect when no users registered
- i18n (en-US / zh-CN) for auth surfaces
- Typed auth API client; parseAuthError unwraps FastAPI detail envelope

Infra & tooling
- Unified `serve.sh` with gateway mode + auto dep install
- Public PyPI uv.toml pin for CI compatibility
- Regenerated uv.lock with public index

Tests
- HTTP vs HTTPS cookie security tests
- Auth middleware, rate limiter, CSRF, setup flow coverage

backend/app/gateway/auth/repositories/base.py

@@ -0,0 +1,82 @@
+"""User repository interface for abstracting database operations."""
+
+from abc import ABC, abstractmethod
+
+from app.gateway.auth.models import User
+
+
+class UserRepository(ABC):
+    """Abstract interface for user data storage.
+
+    Implement this interface to support different storage backends
+    (SQLite)
+    """
+
+    @abstractmethod
+    async def create_user(self, user: User) -> User:
+        """Create a new user.
+
+        Args:
+            user: User object to create
+
+        Returns:
+            Created User with ID assigned
+
+        Raises:
+            ValueError: If email already exists
+        """
+        ...
+
+    @abstractmethod
+    async def get_user_by_id(self, user_id: str) -> User | None:
+        """Get user by ID.
+
+        Args:
+            user_id: User UUID as string
+
+        Returns:
+            User if found, None otherwise
+        """
+        ...
+
+    @abstractmethod
+    async def get_user_by_email(self, email: str) -> User | None:
+        """Get user by email.
+
+        Args:
+            email: User email address
+
+        Returns:
+            User if found, None otherwise
+        """
+        ...
+
+    @abstractmethod
+    async def update_user(self, user: User) -> User:
+        """Update an existing user.
+
+        Args:
+            user: User object with updated fields
+
+        Returns:
+            Updated User
+        """
+        ...
+
+    @abstractmethod
+    async def count_users(self) -> int:
+        """Return total number of registered users."""
+        ...
+
+    @abstractmethod
+    async def get_user_by_oauth(self, provider: str, oauth_id: str) -> User | None:
+        """Get user by OAuth provider and ID.
+
+        Args:
+            provider: OAuth provider name (e.g. 'github', 'google')
+            oauth_id: User ID from the OAuth provider
+
+        Returns:
+            User if found, None otherwise
+        """
+        ...