docs: Ledger API reference page at /docs/api

Sweet asked for API docs today (Slack, 2026-07-14).

## Scope
- New route `/docs/api` in the (app) group, gated to authenticated users.
- Auto-generated from the routes tree + Zod schemas in `apps/web/src/lib/schemas.ts`.
- For each endpoint under `/api/studio/[team]/...`: method, path, auth requirement (bearer vs session), request Zod shape, response shape, copy-paste curl example.
- Include the bearer-provisioning steps: env var pattern `LEDGER_BEARER_<TEAM_UPPER>`, `vercel env pull` recipe.
- Link to `/docs/api` from the settings nav.

## Verification
- Sweet opens /docs/api and closes her Slack question without further help.