feat(auth): 实现 TOTP 双因素认证功能

新增功能：
- 支持 Google Authenticator 等应用进行 TOTP 二次验证
- 用户可在个人设置中启用/禁用 2FA
- 登录时支持 TOTP 验证流程
- 管理后台可全局开关 TOTP 功能

安全增强：
- TOTP 密钥使用 AES-256-GCM 加密存储
- 添加 TOTP_ENCRYPTION_KEY 配置项，必须手动配置才能启用功能
- 防止服务重启导致加密密钥变更使用户无法登录
- 验证失败次数限制，防止暴力破解

配置说明：
- Docker 部署：在 .env 中设置 TOTP_ENCRYPTION_KEY
- 非 Docker 部署：在 config.yaml 中设置 totp.encryption_key
- 生成密钥命令：openssl rand -hex 32

frontend/src/components/user/profile/ProfileTotpCard.vue

@@ -0,0 +1,154 @@
+<template>
+  <div class="card">
+    <div class="border-b border-gray-100 px-6 py-4 dark:border-dark-700">
+      <h2 class="text-lg font-medium text-gray-900 dark:text-white">
+        {{ t('profile.totp.title') }}
+      </h2>
+      <p class="mt-1 text-sm text-gray-500 dark:text-gray-400">
+        {{ t('profile.totp.description') }}
+      </p>
+    </div>
+    <div class="px-6 py-6">
+      <!-- Loading state -->
+      <div v-if="loading" class="flex items-center justify-center py-8">
+        <div class="animate-spin rounded-full h-8 w-8 border-b-2 border-primary-500"></div>
+      </div>
+
+      <!-- Feature disabled globally -->
+      <div v-else-if="status && !status.feature_enabled" class="flex items-center gap-4 py-4">
+        <div class="flex-shrink-0 rounded-full bg-gray-100 p-3 dark:bg-dark-700">
+          <svg class="h-6 w-6 text-gray-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="1.5">
+            <path stroke-linecap="round" stroke-linejoin="round" d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126zM12 15.75h.007v.008H12v-.008z" />
+          </svg>
+        </div>
+        <div>
+          <p class="font-medium text-gray-700 dark:text-gray-300">
+            {{ t('profile.totp.featureDisabled') }}
+          </p>
+          <p class="text-sm text-gray-500 dark:text-gray-400">
+            {{ t('profile.totp.featureDisabledHint') }}
+          </p>
+        </div>
+      </div>
+
+      <!-- 2FA Enabled -->
+      <div v-else-if="status?.enabled" class="flex items-center justify-between">
+        <div class="flex items-center gap-4">
+          <div class="flex-shrink-0 rounded-full bg-green-100 p-3 dark:bg-green-900/30">
+            <svg class="h-6 w-6 text-green-600 dark:text-green-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="1.5">
+              <path stroke-linecap="round" stroke-linejoin="round" d="M9 12.75L11.25 15 15 9.75m-3-7.036A11.959 11.959 0 013.598 6 11.99 11.99 0 003 9.749c0 5.592 3.824 10.29 9 11.623 5.176-1.332 9-6.03 9-11.622 0-1.31-.21-2.571-.598-3.751h-.152c-3.196 0-6.1-1.248-8.25-3.285z" />
+            </svg>
+          </div>
+          <div>
+            <p class="font-medium text-gray-900 dark:text-white">
+              {{ t('profile.totp.enabled') }}
+            </p>
+            <p v-if="status.enabled_at" class="text-sm text-gray-500 dark:text-gray-400">
+              {{ t('profile.totp.enabledAt') }}: {{ formatDate(status.enabled_at) }}
+            </p>
+          </div>
+        </div>
+        <button
+          type="button"
+          class="btn btn-outline-danger"
+          @click="showDisableDialog = true"
+        >
+          {{ t('profile.totp.disable') }}
+        </button>
+      </div>
+
+      <!-- 2FA Not Enabled -->
+      <div v-else class="flex items-center justify-between">
+        <div class="flex items-center gap-4">
+          <div class="flex-shrink-0 rounded-full bg-gray-100 p-3 dark:bg-dark-700">
+            <svg class="h-6 w-6 text-gray-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="1.5">
+              <path stroke-linecap="round" stroke-linejoin="round" d="M9 12.75L11.25 15 15 9.75m-3-7.036A11.959 11.959 0 013.598 6 11.99 11.99 0 003 9.749c0 5.592 3.824 10.29 9 11.623 5.176-1.332 9-6.03 9-11.622 0-1.31-.21-2.571-.598-3.751h-.152c-3.196 0-6.1-1.248-8.25-3.285z" />
+            </svg>
+          </div>
+          <div>
+            <p class="font-medium text-gray-700 dark:text-gray-300">
+              {{ t('profile.totp.notEnabled') }}
+            </p>
+            <p class="text-sm text-gray-500 dark:text-gray-400">
+              {{ t('profile.totp.notEnabledHint') }}
+            </p>
+          </div>
+        </div>
+        <button
+          type="button"
+          class="btn btn-primary"
+          @click="showSetupModal = true"
+        >
+          {{ t('profile.totp.enable') }}
+        </button>
+      </div>
+    </div>
+
+    <!-- Setup Modal -->
+    <TotpSetupModal
+      v-if="showSetupModal"
+      @close="showSetupModal = false"
+      @success="handleSetupSuccess"
+    />
+
+    <!-- Disable Dialog -->
+    <TotpDisableDialog
+      v-if="showDisableDialog"
+      @close="showDisableDialog = false"
+      @success="handleDisableSuccess"
+    />
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, onMounted } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { totpAPI } from '@/api'
+import type { TotpStatus } from '@/types'
+import TotpSetupModal from './TotpSetupModal.vue'
+import TotpDisableDialog from './TotpDisableDialog.vue'
+
+const { t } = useI18n()
+
+const loading = ref(true)
+const status = ref<TotpStatus | null>(null)
+const showSetupModal = ref(false)
+const showDisableDialog = ref(false)
+
+const loadStatus = async () => {
+  loading.value = true
+  try {
+    status.value = await totpAPI.getStatus()
+  } catch (error) {
+    console.error('Failed to load TOTP status:', error)
+  } finally {
+    loading.value = false
+  }
+}
+
+const handleSetupSuccess = () => {
+  showSetupModal.value = false
+  loadStatus()
+}
+
+const handleDisableSuccess = () => {
+  showDisableDialog.value = false
+  loadStatus()
+}
+
+const formatDate = (timestamp: number) => {
+  // Backend returns Unix timestamp in seconds, convert to milliseconds
+  const date = new Date(timestamp * 1000)
+  return date.toLocaleDateString(undefined, {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+    hour: '2-digit',
+    minute: '2-digit'
+  })
+}
+
+onMounted(() => {
+  loadStatus()
+})
+</script>