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/featureFlags.js

@@ -42,7 +42,6 @@ export const FEATURE_FLAGS = {
   CAPTAIN_V2: 'captain_integration_v2',
   CAPTAIN_TASKS: 'captain_tasks',
   CAPTAIN_DOCUMENT_AUTO_SYNC: 'captain_document_auto_sync',
-  SAML: 'saml',
   COMPANIES: 'companies',
   ADVANCED_SEARCH: 'advanced_search',
   CONVERSATION_REQUIRED_ATTRIBUTES: 'conversation_required_attributes',
@@ -56,7 +55,6 @@ export const PREMIUM_FEATURES = [
   FEATURE_FLAGS.CUSTOM_ROLES,
   FEATURE_FLAGS.AUDIT_LOGS,
   FEATURE_FLAGS.HELP_CENTER,
-  FEATURE_FLAGS.SAML,
   FEATURE_FLAGS.CONVERSATION_REQUIRED_ATTRIBUTES,
   FEATURE_FLAGS.ADVANCED_ASSIGNMENT,
 ];