feat(backend): tolerate Authentik users without an email + verify against real SSO

Authentik accounts don't always have an email set (the test account
`hermes` doesn't). AuthentikStrategy / verifyAuthentikClaims no longer
reject those — `authentikEmail()` falls back to a stable
`<preferred_username|sub>@no-email.authentik` handle for the local User row,
and first/last name fall back to preferred_username/name.

Set AUTHENTIK_LEITUNGSTEAM_GROUP to the real group "KC-APP-LT".

Verified end to end against the live https://sso.konfi-castle.com with a
password-grant token for a KC-APP-LT member: backend accepts the RS256
token (JWKS + trailing-slash issuer), JIT-provisions the User, maps the
`groups` claim to isLeitungsteam=true, and POST /api/kc returns 201. Only
the in-browser redirect round-trip remains untested.

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

backend/src/auth/token-verification.service.ts

@@ -10,6 +10,7 @@ import { GuestJwtPayload } from './guest-auth.service';
 import { TeamAuthService } from './team-auth.service';
 import {
   AuthentikClaims,
+  authentikEmail,
   resolveOrProvisionAuthentikUser,
   toAuthenticatedUser,
 } from './provision-user';
@@ -55,15 +56,22 @@ export class TokenVerificationService {
       email?: string;
       given_name?: string;
       family_name?: string;
+      preferred_username?: string;
+      name?: string;
       groups?: string[];
     };
-    if (!payload.sub || !payload.email) {
-      throw new UnauthorizedException('Authentik token missing subject or email');
+    const sub = payload.sub;
+    if (!sub) {
+      throw new UnauthorizedException('Authentik token missing subject');
     }
     return {
-      sub: payload.sub,
-      email: payload.email,
-      firstName: payload.given_name ?? '',
+      sub,
+      email: authentikEmail({
+        email: payload.email,
+        preferred_username: payload.preferred_username,
+        sub,
+      }),
+      firstName: payload.given_name ?? payload.preferred_username ?? payload.name ?? '',
       lastName: payload.family_name ?? '',
       isLeitungsteam: (payload.groups ?? []).includes(this.leitungsteamGroup),
     };