feat(backend): local accounts + invites for Gemeinde Teamer

Per the updated plan, Gemeinde Teamer are no longer Authentik-backed; they
are local accounts a Gemeinde Verantwortliche/r provisions per KC.

Schema:
- User.authentikSub now nullable; add passwordHash + kcId (cascade from Kc)
  so one User model covers Authentik members and local Teamer.
- new TeamerInvite model: shareable group link (email null, maxUses null)
  or personal invite (email pinned, single use), with expiry + revoke.
- sync log now also replicates User / Membership / TeamerInvite.

Auth:
- TeamAuthService: bcrypt password login (POST /auth/team-login) and invite
  redemption (POST /auth/teamer/register) issuing a JWT signed with
  TEAM_JWT_SECRET, payload typ:"team".
- TeamJwtStrategy (AuthGuard('team')) resolves it to the same
  AuthenticatedUser shape as AuthentikStrategy.
- TokenVerificationService.verifyEither() also accepts team tokens (WS).
- files + chat read endpoints accept 'team' tokens; Teamer see non-Konfi
  files and can use chat / start DMs.

Teamer admin (teamer/ module, under /gemeinde/:gemeindeId):
- POST/GET teamer, DELETE teamer/:userId
- POST/GET teamer-invites, DELETE teamer-invites/:inviteId
- LT may manage any Gemeinde; a Verantwortliche/r only their own
  (checked in TeamerService, since RolesGuard only scopes by kcId).

Tests: TeamAuthService + TeamerService specs added (Prisma/Sync mocked),
npm test green at 32. Docs (plan + backend README) updated.

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

backend/.env.example

@@ -7,6 +7,9 @@ AUTHENTIK_ISSUER_URL="https://authentik.example.org/application/o/kc-app"
 # Secret used to sign guest/Konfi session tokens (local accounts only)
 GUEST_JWT_SECRET="change-me"
 
+# Secret used to sign local Gemeinde Teamer session tokens (password login)
+TEAM_JWT_SECRET="change-me-too"
+
 PORT=3000
 
 # File storage: defaults to Nextcloud via WebDAV; set STORAGE_PROVIDER=s3 to