Get canonical plan quotas, pricing, and features

Single source of truth for plan quotas, pricing, and feature lists.

Public, unauthenticated endpoint - the website, customer dashboard, and transactional email copy should fetch this instead of hardcoding plan numbers, so they can't silently drift from what the API actually enforces. See docs/config-centralization-audit-2026-06-22.md.

Also includes watched_companies_limit - the number of distinct companies a plan may watch via webhooks (null = unlimited). A key's individual limit may be higher than its plan default if the Compliance Monitoring add-on is active (see POST /v1/webhooks).

Example: GET /v1/plans