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/errors.py

@@ -0,0 +1,44 @@
+"""Typed error definitions for auth module.
+
+AuthErrorCode: exhaustive enum of all auth failure conditions.
+TokenError: exhaustive enum of JWT decode failures.
+AuthErrorResponse: structured error payload for HTTP responses.
+"""
+
+from enum import StrEnum
+
+from pydantic import BaseModel
+
+
+class AuthErrorCode(StrEnum):
+    """Exhaustive list of auth error conditions."""
+
+    INVALID_CREDENTIALS = "invalid_credentials"
+    TOKEN_EXPIRED = "token_expired"
+    TOKEN_INVALID = "token_invalid"
+    USER_NOT_FOUND = "user_not_found"
+    EMAIL_ALREADY_EXISTS = "email_already_exists"
+    PROVIDER_NOT_FOUND = "provider_not_found"
+    NOT_AUTHENTICATED = "not_authenticated"
+
+
+class TokenError(StrEnum):
+    """Exhaustive list of JWT decode failure reasons."""
+
+    EXPIRED = "expired"
+    INVALID_SIGNATURE = "invalid_signature"
+    MALFORMED = "malformed"
+
+
+class AuthErrorResponse(BaseModel):
+    """Structured error response — replaces bare `detail` strings."""
+
+    code: AuthErrorCode
+    message: str
+
+
+def token_error_to_code(err: TokenError) -> AuthErrorCode:
+    """Map TokenError to AuthErrorCode — single source of truth."""
+    if err == TokenError.EXPIRED:
+        return AuthErrorCode.TOKEN_EXPIRED
+    return AuthErrorCode.TOKEN_INVALID