B2B-6 · Member invitations (org-scoped)

Part of the B2B epic (Fase 1 · item 1.3). Invitations hang off the Organization (B2B-5), not the namespace.

## What
- [ ] `Invite` schema (email / org / role / token / expiry)
- [ ] `POST /organization/:id/invite` endpoint + e-mail (Nodemailer + Handlebars templates already exist)
- [ ] Wire token → registration with pre-set role/org — reuse Ory recovery links or own token (`server/users/users.controller.ts:67-88` self-service register; `ory.service.ts:102-137` `createIdentity` already in use)
- [ ] Frontend: **`src/pages/signup-invite.tsx` + `RegistrationInvite/` already exist but are non-functional** — wire them up

**Effort:** ~1–1.5 SP · **Depends on:** B2B-5