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.

frontend/src/pages/api-docs/endpoints.ts

@@ -307,6 +307,11 @@ export const sections: readonly Section[] = [
         path: '/panel/api/server/getDb',
         summary: 'Stream the SQLite database file as an attachment. Use as a manual backup.',
       },
+      {
+        method: 'GET',
+        path: '/panel/api/server/getMigration',
+        summary: 'Stream a cross-engine migration file as an attachment: a .dump (SQL text) on SQLite, or a .db SQLite database built from the live data on PostgreSQL.',
+      },
       {
         method: 'GET',
         path: '/panel/api/server/getNewUUID',