Add an entitlement

Plans and what they allow are in packages/contracts/src/billing.ts; Stripe holds the prices. With billing off, every organization has everything (unlimited). docs/files-and-billing.md, "Plans and entitlements", has the model.

1. Declare it in Entitlements, and set it in every plan in plans and in unlimited (false/true for a feature, a number or null for a limit). The satisfies clauses fail check-types until each has it.
2. Enforce it where the feature is used, in the API: await this.billing.require(orgId, "<entitlement>") (BillingService, apps/api), which throws ENTITLEMENT_REQUIRED. A limit is checked where the count grows, like members in apps/api/src/auth/auth-plugins.ts.
3. Show it in the clients from billing.overview: hide the feature or show UpgradeHint (apps/web/src/modules/billing/components/upgrade-hint.tsx), with its copy in every catalog.
4. Test in apps/api/test/billing.integration.test.ts (against the fake Stripe): a free organization gets ENTITLEMENT_REQUIRED, a paying one gets through, and with billing off everyone does.
5. Update the plans table in docs/files-and-billing.md.

Done when

bun run check-types, bun run test and bun run test:integration pass, and the reviewer agent reports nothing blocking.