B2B-8 · Minimal manual billing (plan/tier on Organization)

Part of the B2B epic (Fase 1 · item 1.5). Just enough to invoice 1–3 clients — payment gateway and invoicing stay **out of product** in this phase.

## What
- [ ] `plan/tier` field on `Organization` (`free/contributor/paid` — same field the contribution gate B2B-16 reads)
- [ ] Manual provisioning: admin toggle endpoint/UI

Real recurring billing (Asaas/Iugu/PIX + NF) is a separate later epic — do not scope it here.

**Effort:** ~0.5 SP · **Depends on:** B2B-5

Start by reading the dependency B2B-5 and locating the Organization model, then inspect the contribution gate referenced by B2B-16 and the existing admin endpoint and UI patterns. Done means Organization supports free, contributor, and paid plan/tier values and an administrator can manually change them; recurring gateway and invoicing work remains out of scope.

索引モデルが issue の本文から書いたものです。