feat: add multi-provider auth via AuthIdentity and extend site config

- Extract AuthIdentity model for multi-provider authentication (email_password, OAuth, Passkey, Magic Link)
- Remove password field from User model, credentials now stored in AuthIdentity
- Refactor unified login/register to use AuthIdentity-based provider checking
- Add site config fields: footer_code, tos_url, privacy_url, auth_methods
- Add auth settings defaults in migration (email_password enabled by default)
- Update admin user creation to create AuthIdentity records
- Update all tests to use AuthIdentity model

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sqlmodels/__init__.py

@@ -1,9 +1,16 @@
+from .auth_identity import (
+    AuthIdentity,
+    AuthIdentityResponse,
+    AuthProviderType,
+    BindIdentityRequest,
+)
 from .user import (
     BatchDeleteRequest,
     JWTPayload,
-    LoginRequest,
+    MagicLinkRequest,
+    UnifiedLoginRequest,
+    UnifiedRegisterRequest,
     RefreshTokenRequest,
-    RegisterRequest,
     AccessTokenBase,
     RefreshTokenBase,
     TokenResponse,
@@ -89,7 +96,7 @@ from .policy import Policy, PolicyBase, PolicyOptions, PolicyOptionsBase, Policy
 from .redeem import Redeem, RedeemType
 from .report import Report, ReportReason
 from .setting import (
-    Setting, SettingsType, SiteConfigResponse,
+    Setting, SettingsType, SiteConfigResponse, AuthMethodConfig,
     # 管理员DTO
     SettingItem, SettingsListResponse, SettingsUpdateRequest, SettingsUpdateResponse,
 )
@@ -120,4 +127,4 @@ from .model_base import (
 )
 
 # mixin 中的通用分页模型
-from .mixin import ListResponse
+from .mixin import ListResponse