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-TW.json

@@ -277,7 +277,10 @@
       "getConfigError": "檢索設定檔時發生錯誤",
       "backupPostgresNote": "此面板執行於 PostgreSQL 上。「備份」會下載一個 pg_dump 封存檔（.dump），「還原」會透過 pg_restore 重新載入。伺服器需要安裝 PostgreSQL 用戶端工具（pg_dump 與 pg_restore）。",
       "exportDatabasePgDesc": "點擊將目前資料庫的 PostgreSQL 傾印（.dump）下載到您的裝置。",
-      "importDatabasePgDesc": "點擊選擇並上傳 .dump 檔案以還原您的 PostgreSQL 資料庫。此操作將取代所有目前的資料。"
+      "importDatabasePgDesc": "點擊選擇並上傳 .dump 檔案以還原您的 PostgreSQL 資料庫。此操作將取代所有目前的資料。",
+      "migrationDownload": "下載遷移檔案",
+      "migrationDownloadDesc": "點擊下載目前 SQLite 資料庫的可攜式 .dump（SQL 文字）匯出檔。",
+      "migrationDownloadPgDesc": "點擊下載由 PostgreSQL 資料建立的 .db SQLite 資料庫，可用於在 SQLite 上執行本面板。"
     },
     "inbounds": {
       "title": "入站",