feat: public usage endpoint GET /v1/usage for member token metrics #102

Members can currently only see their token usage through the cloud-ui dashboard. This feature adds a public, documented, rate-limited GET /v1/usage on api.nan.builders so they (and their own tooling) can query their usage programmatically.

Scope

• cloud-api: new public route with OpenAI-style auth (Bearer sk-), member-level numbers matching the dashboard, window capped at 90 inclusive days, cursor pagination, per-member rate limit (30 rpm), 60s snapshot cache, all-time block.
• website: /usage documented in the Scalar-rendered OpenAPI spec + getting-started guides (EN/ES).

Decisions (user-confirmed)

• Hybrid OpenAI-like envelope (usage.report), member-level scope, 90-day window + all_time block.

Tracking

• cloud-api PR: helmcode/nan-cloud-api
• website PR: helmcode/nan
• Reviews: Security / UX / QA blocked until APPROVED (Opus rounds: security 2x APPROVED, ux 2x APPROVED, qa 3 rounds -> APPROVED).