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/models.py

@@ -0,0 +1,41 @@
+"""User Pydantic models for authentication."""
+
+from datetime import UTC, datetime
+from typing import Literal
+from uuid import UUID, uuid4
+
+from pydantic import BaseModel, ConfigDict, EmailStr, Field
+
+
+def _utc_now() -> datetime:
+    """Return current UTC time (timezone-aware)."""
+    return datetime.now(UTC)
+
+
+class User(BaseModel):
+    """Internal user representation."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: UUID = Field(default_factory=uuid4, description="Primary key")
+    email: EmailStr = Field(..., description="Unique email address")
+    password_hash: str | None = Field(None, description="bcrypt hash, nullable for OAuth users")
+    system_role: Literal["admin", "user"] = Field(default="user")
+    created_at: datetime = Field(default_factory=_utc_now)
+
+    # OAuth linkage (optional)
+    oauth_provider: str | None = Field(None, description="e.g. 'github', 'google'")
+    oauth_id: str | None = Field(None, description="User ID from OAuth provider")
+
+    # Auth lifecycle
+    needs_setup: bool = Field(default=False, description="True for auto-created admin until setup completes")
+    token_version: int = Field(default=0, description="Incremented on password change to invalidate old JWTs")
+
+
+class UserResponse(BaseModel):
+    """Response model for user info endpoint."""
+
+    id: str
+    email: str
+    system_role: Literal["admin", "user"]
+    needs_setup: bool = False