FAL veo3.1 API expects duration as "4s"/"6s"/"8s" (with unit suffix),
not bare "4"/"6"/"8" like other families. Add per-family duration_suffix
field and apply it in _build_payload. Also add "4k" to veo3.1 resolutions
per FAL API docs.
Note: the managed gateway currently rejects the "4s" format (expects
integer duration). Gateway-side fix needed for veo3.1 to work through
the Nous subscription path.