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.
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