Lets operators with large client counts or multi-node setups pick PostgreSQL
at install time without breaking the existing SQLite default. Backend is
selected at runtime via XUI_DB_TYPE/XUI_DB_DSN, a small dialect layer keeps
the five JSON_EXTRACT/JSON_EACH queries portable, and a new `x-ui migrate-db`
subcommand copies SQLite data into PostgreSQL in FK-aware order.
