Model marketplace

Integrate Half Off Dev models (Claude, GPT, Gemini and more, one prepaid key) into this project.

- Docs (read first): https://halfoff.dev/docs/models.md and https://halfoff.dev/docs/api/models.md
- Each model family keeps its own native API, no translation: Anthropic Messages at https://halfoff.dev/v1/messages (Anthropic SDK, base URL https://halfoff.dev), OpenAI Chat Completions / Responses at https://halfoff.dev/v1 (OpenAI SDK), Gemini at https://halfoff.dev/v1beta (Google GenAI SDK, base URL https://halfoff.dev). Use the SDK for the model family you pick.
- GET https://halfoff.dev/v1/models lists every model id with its price and API formats; pick one there, do not guess ids.
- Auth: read my API key from the HALFOFF_API_KEY environment variable and send it as `Authorization: Bearer $HALFOFF_API_KEY`. Never print or commit the key. If it is not set, ask me to create one at https://halfoff.dev/app/keys.
- Billing is prepaid: every call is charged to my balance, so test with a small request before running anything large.
- Every docs page is plain Markdown; the full index is https://halfoff.dev/llms.txt.

My task: <describe what you want to build or get>