Submit async DuckDB job - returns immediately with job ID for polling

Compute resource request shared by the DuckDB cluster and E2B sandbox paths.

Defaults are sized for the DuckDB cluster. Non-DuckDB callers must check model_fields_set before reading cpu/memory, otherwise the DuckDB defaults will be used for other engines — see defapi.config.e2b.select_e2b_sandbox_tier for an example.