feat(genai): add TwelveLabs Marengo embeddings provider

Adds an opt-in 'twelvelabs' GenAI provider implementing the embeddings
role using TwelveLabs' Marengo multimodal model. Text and image inputs
are embedded into a shared vector space, powering semantic search over
event thumbnails.

The provider only implements the embeddings role; descriptions/chat are
left to other providers since Marengo is an embedding model. Marengo
returns 512-dim vectors, which the existing GenAIEmbedding adapter pads
to Frigate's 768-dim search schema.

Includes no-network unit tests and a live test gated on
TWELVELABS_API_KEY.

docker/main/requirements-wheels.txt

@@ -50,6 +50,7 @@ transformers == 4.45.*
 google-genai == 1.58.*
 ollama == 0.6.*
 openai == 1.65.*
+twelvelabs == 1.2.*
 # push notifications
 py-vapid == 1.9.*
 pywebpush == 2.0.*