Allow Pro/Max accounts to issue API keys (unified billing)

Pro/Max subscribers can't currently use their subscription with the Anthropic API — only via Claude Code. Request: let Pro/Max accounts mint API keys that draw from the subscription's included tokens first, then fall back to metered usage on the same payment method.

Why: Devs end up maintaining two billing surfaces (Pro/Max subscription + separate API key with its own card and budget). One account, one bill, one usage dashboard would simplify ops significantly — especially for solo devs and small teams building SDK-based tools alongside their CC usage.

Ideal: API requests consume subscription tokens until exhausted, then bill metered.

Acceptable fallback: API key just bills metered against the same account, but lives in the same dashboard alongside CC usage.