feat(auth): introduce backend auth module

Port RFC-001 authentication core from PR #1728:
- JWT token handling (create_access_token, decode_token, TokenPayload)
- Password hashing (bcrypt) with verify_password
- SQLite UserRepository with base interface
- Provider Factory pattern (LocalAuthProvider)
- CLI reset_admin tool
- Auth-specific errors (AuthErrorCode, TokenError, AuthErrorResponse)

Deps:
- bcrypt>=4.0.0
- pyjwt>=2.9.0
- email-validator>=2.0.0
- backend/uv.toml pins public PyPI index

Tests: 12 pure unit tests (test_auth_config.py, test_auth_errors.py).

Scope note: authz.py, test_auth.py, and test_auth_type_system.py are
deferred to commit 2 because they depend on middleware and deps wiring
that is not yet in place. Commit 1 stays "pure new files only" as the
spec mandates.

backend/app/gateway/auth/__init__.py

@@ -0,0 +1,42 @@
+"""Authentication module for DeerFlow.
+
+This module provides:
+- JWT-based authentication
+- Provider Factory pattern for extensible auth methods
+- UserRepository interface for storage backends (SQLite)
+"""
+
+from app.gateway.auth.config import AuthConfig, get_auth_config, set_auth_config
+from app.gateway.auth.errors import AuthErrorCode, AuthErrorResponse, TokenError
+from app.gateway.auth.jwt import TokenPayload, create_access_token, decode_token
+from app.gateway.auth.local_provider import LocalAuthProvider
+from app.gateway.auth.models import User, UserResponse
+from app.gateway.auth.password import hash_password, verify_password
+from app.gateway.auth.providers import AuthProvider
+from app.gateway.auth.repositories.base import UserRepository
+
+__all__ = [
+    # Config
+    "AuthConfig",
+    "get_auth_config",
+    "set_auth_config",
+    # Errors
+    "AuthErrorCode",
+    "AuthErrorResponse",
+    "TokenError",
+    # JWT
+    "TokenPayload",
+    "create_access_token",
+    "decode_token",
+    # Password
+    "hash_password",
+    "verify_password",
+    # Models
+    "User",
+    "UserResponse",
+    # Providers
+    "AuthProvider",
+    "LocalAuthProvider",
+    # Repository
+    "UserRepository",
+]