feat(backend): self-registration for Gemeinde Verantwortliche

New onboarding/ module. A prospective Verantwortliche/r signs in with their
Konfi-Castle-ID (Authentik), looks up a KC by invite code, picks an existing
Gemeinde, and registers:

- GET  /api/onboarding/kc/:inviteCode  -> KC name + its Gemeinden (public;
  the invite code is the shared secret)
- POST /api/onboarding/verantwortliche -> verifies the raw Authentik bearer
  token's claims (no local Membership required yet via new
  TokenVerificationService.verifyAuthentikClaims), JIT-provisions the local
  User, and creates a Membership with status PENDING. Idempotent per
  (user, kc, gemeinde).
- GET  /api/onboarding/requests?kcId=            (LT) list pending
- POST /api/onboarding/requests/:id/approve|reject (LT) approve flips to
  ACTIVE, reject deletes.

Schema: Membership gains status (enum MembershipStatus { ACTIVE, PENDING },
default ACTIVE). AuthentikStrategy / TokenVerificationService / TeamAuthService
now load only ACTIVE memberships, so a pending request grants nothing until
approved. Membership create/update/delete flow through the sync log.

Tests: onboarding.service.spec.ts (14 cases); npm test green at 46.
Docs (plan + backend README) updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/auth/authentik.strategy.ts

@@ -43,7 +43,7 @@ export class AuthentikStrategy extends PassportStrategy(Strategy, 'authentik') {
   async validate(payload: AuthentikJwtPayload): Promise<AuthenticatedUser> {
     const user = await this.prisma.user.findUnique({
       where: { authentikSub: payload.sub },
-      include: { memberships: true },
+      include: { memberships: { where: { status: 'ACTIVE' } } },
     });
     if (!user) {
       throw new UnauthorizedException('User not provisioned locally yet');