Add API user guide for script writers

[ForisKuang]

What

Documentation only; no code changes.

• New docs/api.md: a user guide for MSK staff calling ACGC from scripts.
  • Getting an API key from the /api-keys page, and the key rules: expiry, revoking, 60 requests a minute, and keys can't manage keys.
  • A first request, then each /v1/genes/* endpoint with its request body and every response field.
  • Background jobs, how caching and force_refresh affect speed, and every error code with what to do about it, including the per-gene errors in batch responses.
  • A complete Python example that runs a job, checks on it and handles 429.
• README links:
  • A pointer to the guide near the top.
  • Links at the start of the "ACGC API Keys (for scripts)" and "Gene query API" sections.
  • The old ## API Keys heading, which is about outside services ACGC itself uses (Anthropic, NCBI…), is renamed to ## Third-Party API Keys (used by ACGC) and points script writers to the right place. Nothing in the repo linked to the old #api-keys anchor.

Accuracy

Field names, limits and status codes were checked against the code on main:

• src/api/gene_query.py models;
• src/api_keys_routes.py;
• require_auth in src/auth.py (401 for a bad, revoked or expired key, or an owner no longer on the allowlist; 403 for an account pending approval; 503 when keys can't be verified;
• the README sections from Add slim gene query API with links to the full run #107, Add API-key authentication for scripted ACGC access #108, Add in-app API keys page (/api-keys) #113 and Fix gene API persistence and HGNC lookup failures #114.
  EOF
  )