fix(video_gen): veo3.1 duration format and 4k resolution

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.

tests/plugins/video_gen/test_fal_plugin.py

@@ -257,7 +257,7 @@ class TestPayloadBuilder:
             seed=42,
         )
         assert p["prompt"] == "x"
-        assert p["duration"] == "8"  # FAL queue API uses strings
+        assert p["duration"] == "8s"  # veo3.1 uses "Ns" format per FAL API
         assert p["aspect_ratio"] == "16:9"
         assert p["resolution"] == "720p"
         assert p["generate_audio"] is True