feat: 数据库定时备份与恢复（S3 兼容存储，支持 Cloudflare R2）

新增管理员专属的数据库备份与恢复功能：
- 全量 PostgreSQL 备份（pg_dump），gzip 压缩后上传到 S3 兼容存储
- 支持手动备份和 cron 定时备份
- 支持从备份恢复（psql --single-transaction）
- 备份文件自动过期清理（默认 14 天）
- 前端完整管理页面（S3 配置、定时配置、备份列表、恢复/下载/删除）
- 内置 Cloudflare R2 配置教程弹窗
- Dockerfile 从 postgres 镜像多阶段复制 pg_dump/psql，确保版本一致

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

frontend/src/i18n/locales/en.ts

@@ -340,6 +340,7 @@ export default {
     redeemCodes: 'Redeem Codes',
     ops: 'Ops',
     promoCodes: 'Promo Codes',
+    backup: 'DB Backup',
     dataManagement: 'Data Management',
     settings: 'Settings',
     myAccount: 'My Account',
@@ -966,6 +967,110 @@ export default {
       failedToLoad: 'Failed to load dashboard statistics'
     },
 
+    backup: {
+      title: 'Database Backup',
+      description: 'Full database backup to S3-compatible storage with scheduled backup and restore',
+      s3: {
+        title: 'S3 Storage Configuration',
+        description: 'Configure S3-compatible storage (supports Cloudflare R2)',
+        descriptionPrefix: 'Configure S3-compatible storage (supports',
+        descriptionSuffix: ')',
+        enabled: 'Enable S3 Storage',
+        endpoint: 'Endpoint',
+        region: 'Region',
+        bucket: 'Bucket',
+        prefix: 'Key Prefix',
+        accessKeyId: 'Access Key ID',
+        secretAccessKey: 'Secret Access Key',
+        secretConfigured: 'Already configured, leave empty to keep',
+        forcePathStyle: 'Force Path Style',
+        testConnection: 'Test Connection',
+        testSuccess: 'S3 connection test successful',
+        testFailed: 'S3 connection test failed',
+        saved: 'S3 configuration saved'
+      },
+      schedule: {
+        title: 'Scheduled Backup',
+        description: 'Configure automatic scheduled backups',
+        enabled: 'Enable Scheduled Backup',
+        cronExpr: 'Cron Expression',
+        cronHint: 'e.g. "0 2 * * *" means every day at 2:00 AM',
+        retainDays: 'Backup Expire Days',
+        retainDaysHint: 'Backup files auto-delete after this many days, 0 = never expire',
+        retainCount: 'Max Retain Count',
+        retainCountHint: 'Maximum number of backups to keep, 0 = unlimited',
+        saved: 'Schedule configuration saved'
+      },
+      operations: {
+        title: 'Backup Records',
+        description: 'Create manual backups and manage existing backup records',
+        createBackup: 'Create Backup',
+        backing: 'Backing up...',
+        backupCreated: 'Backup created successfully',
+        expireDays: 'Expire Days'
+      },
+      columns: {
+        status: 'Status',
+        fileName: 'File Name',
+        size: 'Size',
+        expiresAt: 'Expires At',
+        triggeredBy: 'Triggered By',
+        startedAt: 'Started At',
+        actions: 'Actions'
+      },
+      status: {
+        pending: 'Pending',
+        running: 'Running',
+        completed: 'Completed',
+        failed: 'Failed'
+      },
+      trigger: {
+        manual: 'Manual',
+        scheduled: 'Scheduled'
+      },
+      neverExpire: 'Never',
+      empty: 'No backup records',
+      actions: {
+        download: 'Download',
+        restore: 'Restore',
+        restoreConfirm: 'Are you sure you want to restore from this backup? This will overwrite the current database!',
+        restoreSuccess: 'Database restored successfully',
+        deleteConfirm: 'Are you sure you want to delete this backup?',
+        deleted: 'Backup deleted'
+      },
+      r2Guide: {
+        title: 'Cloudflare R2 Setup Guide',
+        intro: 'Cloudflare R2 provides S3-compatible object storage with a free tier of 10GB storage + 1M Class A requests/month, ideal for database backups.',
+        step1: {
+          title: 'Create an R2 Bucket',
+          line1: 'Log in to the Cloudflare Dashboard (dash.cloudflare.com), select "R2 Object Storage" from the sidebar',
+          line2: 'Click "Create bucket", enter a name (e.g. sub2api-backups), choose a region',
+          line3: 'Click create to finish'
+        },
+        step2: {
+          title: 'Create an API Token',
+          line1: 'On the R2 page, click "Manage R2 API Tokens" in the top right',
+          line2: 'Click "Create API token", set permission to "Object Read & Write"',
+          line3: 'Recommended: restrict to specific bucket for better security',
+          line4: 'After creation, you will see the Access Key ID and Secret Access Key',
+          warning: 'The Secret Access Key is only shown once — copy and save it immediately!'
+        },
+        step3: {
+          title: 'Get the S3 Endpoint',
+          desc: 'Find your Account ID on the R2 overview page (in the URL or the right panel). The endpoint format is:',
+          accountId: 'your_account_id'
+        },
+        step4: {
+          title: 'Fill in the Configuration',
+          checkEnabled: 'Checked',
+          bucketValue: 'Your bucket name',
+          fromStep2: 'Value from Step 2',
+          unchecked: 'Unchecked'
+        },
+        freeTier: 'R2 Free Tier: 10GB storage + 1M Class A requests + 10M Class B requests per month — more than enough for database backups.'
+      }
+    },
+
     dataManagement: {
       title: 'Data Management',
       description: 'Manage data management agent status, object storage settings, and backup jobs in one place',