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>

tests/integration/api/test_site.py

@@ -83,6 +83,24 @@ async def test_site_config_captcha_settings(async_client: AsyncClient):
     assert "forgetCaptcha" in config
 
 
+@pytest.mark.asyncio
+async def test_site_config_auth_methods(async_client: AsyncClient):
+    """测试配置包含认证方式列表"""
+    response = await async_client.get("/api/site/config")
+    assert response.status_code == 200
+
+    data = response.json()
+    config = data["data"]
+    assert "authMethods" in config
+    assert isinstance(config["authMethods"], list)
+    assert len(config["authMethods"]) > 0
+
+    # 每个认证方式应包含 provider 和 isEnabled
+    for method in config["authMethods"]:
+        assert "provider" in method
+        assert "isEnabled" in method
+
+
 @pytest.mark.asyncio
 async def test_site_captcha_endpoint_exists(async_client: AsyncClient):
     """测试验证码端点存在（即使未实现也应返回有效响应）"""