B2B-16 · Contribution-vs-payment gate for AI features

Part of the B2B epic (Bloco 4 · item 4.2). Copilot access is currently gated only by role (`@FactCheckerOnly()` on `copilot-chat.controller.ts:64-77`); `feature-flag.service.ts` passes no org/namespace context to Unleash.

## What
- [ ] Read `plan/tier` from `Organization` (field created in B2B-8: `free/contributor/paid`)
- [ ] Contribution counter per org
- [ ] Gate in the copilot controller: contribute → access, or pay
- [ ] Org context in Unleash flags

## Notes
- With orgs inside `main` (committee groups, B2B-5), the gate also works for non-paying collectives that contribute.
- Open decision: contribution threshold.

**Effort:** ~1.5–2 SP · **Depends on:** B2B-15, B2B-5, B2B-8