refactor: 移除 SAML/LDAP/MFA 登录方式，仅保留本地账号密码和 OIDC

后端移除：
- SAML: auth/saml.go, handler/saml_handler.go, account_saml_settings_handler.go,
  model/account_saml_settings.go, model/saml_idp_config.go, repo/*.go
- LDAP: auth/ldap.go, handler/ldap_handler.go, model/account_ldap_settings.go,
  repo/account_ldap_settings_repo.go
- MFA: auth/mfa.go, handler/mfa_handler.go
- auth_service: 移除 mfaService 依赖、MFARequired 字段、LoginWithMFA 方法
- auth_handler: 移除 LoginMFA handler、MFA 分支逻辑
- bootstrap: 移除 SAML/LDAP/MFA service 初始化和 handler 注册
- sso_middleware: 精简为仅支持 OIDC provider
- router: 移除 SAML/LDAP/MFA 路由注册
- config: 移除 SAMLConfig/LDAPConfig struct 和 defaults

前端移除：
- v3/login: 移除 MFA 验证流程和 SAML 登录入口
- v3/api/auth: 移除 MFA 响应处理
- v3/routes: 移除 SSO login 路由
- dashboard: 移除 MFA 设置页面、SAML 安全设置页面
- i18n: 移除 mfa.json
- featureFlags: 移除 SAML feature flag

.env.example / .env: 移除 SAML/LDAP 配置段

frontend/app/javascript/dashboard/api/mfa.js

@@ -1,28 +0,0 @@
-/* global axios */
-import ApiClient from './ApiClient';
-
-class MfaAPI extends ApiClient {
-  constructor() {
-    super('profile/mfa', { accountScoped: false });
-  }
-
-  enable() {
-    return axios.post(`${this.url}`);
-  }
-
-  verify(otpCode) {
-    return axios.post(`${this.url}/verify`, { otp_code: otpCode });
-  }
-
-  disable(password, { otpCode, backupCode } = {}) {
-    return axios.delete(this.url, {
-      data: { password, otp_code: otpCode, backup_code: backupCode },
-    });
-  }
-
-  regenerateBackupCodes(otpCode) {
-    return axios.post(`${this.url}/backup_codes`, { otp_code: otpCode });
-  }
-}
-
-export default new MfaAPI();

frontend/app/javascript/dashboard/api/samlSettings.js

@@ -1,26 +0,0 @@
-/* global axios */
-import ApiClient from './ApiClient';
-
-class SamlSettingsAPI extends ApiClient {
-  constructor() {
-    super('saml_settings', { accountScoped: true });
-  }
-
-  get() {
-    return axios.get(this.url);
-  }
-
-  create(data) {
-    return axios.post(this.url, { saml_settings: data });
-  }
-
-  update(data) {
-    return axios.put(this.url, { saml_settings: data });
-  }
-
-  delete() {
-    return axios.delete(this.url);
-  }
-}
-
-export default new SamlSettingsAPI();