[codex] Add Hugging Face billing callback

[sirouk]

Summary

• Add Inference-Id response headers using the existing Chutes invocation ID.
• Add POST /partners/huggingface/billing for HF requestIds -> costNanoUsd lookups.
• Scope billing lookups to the authenticated Chutes user and allow read-only billing API-key scopes.

Notes

This keeps HF on direct Chutes infrastructure: inference can use llm.chutes.ai/v1, and billing can use api.chutes.ai/partners/huggingface/billing. Unknown request IDs return 0; known IDs use persisted invocation metrics["b"] converted to integer nano-USD.

Checks

• PATH="$PWD/.venv/bin:$PATH" make lint
• DEBUG=false uv run pytest -q tests/unit/test_huggingface_billing.py
• uv run python -m compileall api/invocation/router.py api/payment/router.py api/main.py api/api_key/schemas.py tests/unit/test_huggingface_billing.py