[Revenue] Implement Plan 4: Paid API Keys for Council/Bridge (from 1000 ideas)

[atomeam]

From 1000 Autonomous First Dollar Ideas master https://app.notion.com/p/7d2d305c39974f0fb37261e95ef4f0c0 and Directive v5.

Goal: Once bridge live, expose paid API keys for /api/* (tasks, leads, billing). One-time $19-99 or usage-based via Stripe. Docs + buy button on apex. Key issuance from existing PR #111 logic + billing webhook.

Autonomy: Extremely high.

Evidence: First paid key purchase in Stripe.

First steps for code agents:

1. Public docs page for API.
2. Buy button wired to Stripe Product/Price.
3. Usage metering in D1.
4. Key lifecycle management.

References: PR #111 billing code. Requires bridge deploy first (Issue #117). Continue execution until revenue.