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.

x-ui.sh

@@ -2690,7 +2690,7 @@ migrate_to_postgres() {
     echo ""
     echo -e "${yellow}This copies your current SQLite data into a PostgreSQL database,${plain}"
     echo -e "${yellow}then switches the panel to PostgreSQL and restarts it.${plain}"
-    echo -e "${yellow}The destination PostgreSQL database must be empty.${plain}"
+    echo -e "${red}Any existing panel tables in the destination will be cleared and overwritten.${plain}"
     confirm "Continue?" "n" || return 0
 
     local dsn="" pg_mode