feat(migrate-db): SQLite <-> .dump conversion and Download Migration in Overview

Binary: extend the migrate-db subcommand with --dump and --restore so a
SQLite database can be exported to a portable SQL text dump and rebuilt from
one, alongside the existing --dsn PostgreSQL copy. Implemented in Go via the
bundled sqlite driver (new database/dump_sqlite.go); no external sqlite3 client
is required. Add ExportPostgresToSQLite (reverse of MigrateData) to build a
SQLite .db from live PostgreSQL data, reusing the shared copyAllModels helper.

Overview: add a "Download Migration" item to Backup & Restore plus a
getMigration endpoint/service that returns a .dump on SQLite or a .db on
PostgreSQL, so the data can seed a panel on the other backend. Document the
endpoint in api-docs and translate the three new strings across all locales.

Tests: cover the destination-side copy (AutoMigrate + copyTable into SQLite)
and the dump/restore round-trip including quoted values. Ignore *.dump.

The x-ui.sh helper that drives this from the CLI is in PR #4910.

web/translation/zh-CN.json

@@ -277,7 +277,10 @@
       "getConfigError": "检索配置文件时出错",
       "backupPostgresNote": "此面板运行在 PostgreSQL 上。「备份」会下载一个 pg_dump 归档（.dump），「恢复」会通过 pg_restore 重新载入。服务器需要安装 PostgreSQL 客户端工具（pg_dump 和 pg_restore）。",
       "exportDatabasePgDesc": "点击将当前数据库的 PostgreSQL 转储（.dump）下载到您的设备。",
-      "importDatabasePgDesc": "点击选择并上传 .dump 文件以恢复您的 PostgreSQL 数据库。此操作将替换所有当前数据。"
+      "importDatabasePgDesc": "点击选择并上传 .dump 文件以恢复您的 PostgreSQL 数据库。此操作将替换所有当前数据。",
+      "migrationDownload": "下载迁移文件",
+      "migrationDownloadDesc": "点击下载当前 SQLite 数据库的可移植 .dump（SQL 文本）导出文件。",
+      "migrationDownloadPgDesc": "点击下载由 PostgreSQL 数据构建的 .db SQLite 数据库，可用于在 SQLite 上运行本面板。"
     },
     "inbounds": {
       "title": "入站",