feat(video_gen): wire Nous subscription override into hermes tools UX

Add the same managed-gateway UX that image_gen already has:

- TOOL_CATEGORIES['video_gen'] gets a 'Nous Subscription' provider row
  with managed_nous_feature='video_gen' + video_gen_plugin_name='fal'
- NousSubscriptionFeatures gains a video_gen property + feature state
  computation (managed/active/available using the fal-queue gateway)
- _GATEWAY_TOOL_LABELS, _GATEWAY_DIRECT_LABELS, _ALL_GATEWAY_KEYS,
  _get_gateway_direct_credentials, opted_in all include video_gen
- apply_nous_managed_defaults and apply_gateway_defaults handle video_gen
- _is_toolset_satisfied checks Nous features for video_gen
- _is_provider_active detects managed video_gen (use_gateway + fal provider)
- _select_plugin_video_gen_provider accepts use_gateway kwarg, propagated
  from all 4 call sites in _configure_provider when managed_feature is set
- hermes setup status shows 'Video Generation (FAL via Nous subscription)'

Users on a Nous subscription can now pick 'Nous Subscription' under
hermes tools → Video Generation, which sets video_gen.provider=fal +
video_gen.use_gateway=true. The FAL plugin's _resolve_managed_fal_video_gateway
then routes through the managed queue gateway — no FAL_KEY needed.

tests/hermes_cli/test_nous_subscription.py

@@ -218,7 +218,7 @@ def test_get_gateway_eligible_tools_ignores_quoted_false_opt_in(monkeypatch):
     monkeypatch.setattr(
         ns,
         "_get_gateway_direct_credentials",
-        lambda: {"web": True, "image_gen": False, "tts": False, "browser": False},
+        lambda: {"web": True, "image_gen": False, "video_gen": False, "tts": False, "browser": False},
     )
 
     unconfigured, has_direct, already_managed = ns.get_gateway_eligible_tools(
@@ -230,4 +230,4 @@ def test_get_gateway_eligible_tools_ignores_quoted_false_opt_in(monkeypatch):
 
     assert "web" in has_direct
     assert "web" not in already_managed
-    assert set(unconfigured) == {"image_gen", "tts", "browser"}
+    assert set(unconfigured) == {"image_gen", "video_gen", "tts", "browser"}