feat: Authentik OIDC login (PKCE) + Leitungsteam admin screens

Backend:
- AuthentikStrategy / TokenVerificationService: normalise the issuer's
  trailing slash and accept both `iss` spellings (Authentik's discovery
  issuer and token `iss` carry a trailing slash; the JWKS URL must not
  double it). Wire the real konfi-castle issuer into .env.example.
- team token path now goes through toAuthenticatedUser too, so a local
  account flagged isLeitungsteam gets the synthetic global LT membership
  regardless of token kind.
- LT-admin controllers (kc, gemeinde, onboarding, sync, teamer) accept
  ['authentik','team'] so such an account can use them. RolesGuard still
  enforces the actual LT/role check.
- app.module serves the Flutter web build from client/app/build/web (SPA
  fallback covers the OIDC redirect path /v1/auth/callback), falling back
  to the interim client/web/ if it isn't built.

Client (client/app/):
- oidc.dart: Authorization-Code + PKCE against Authentik (discovery, S256
  challenge, state, token exchange, refresh). Browser bits (sessionStorage,
  redirect, URL) behind a conditional import so `flutter test` still
  compiles on the VM.
- AppState handles the ?code= callback on bootstrap, stores access +
  refresh, refreshes an expired token on restart.
- Login screen: "Mit Konfi-Castle-ID anmelden" button (Leitungsteam /
  Verantwortliche) alongside the local Teamer password form.
- admin_screen.dart: LT-only "Verwaltung" — list/create KCs, per KC the
  Gemeinden (list/create) and pending Verantwortlichen requests
  (approve/reject). Verified end to end against local Postgres with an
  isLeitungsteam account (create KC/Gemeinde, list + approve a request).

flutter analyze/test/build web green; backend npm test 56.

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

src/auth/team-auth.service.ts

@@ -12,6 +12,7 @@ import * as jwt from 'jsonwebtoken';
 import { PrismaClient } from '../prisma/prisma.module';
 import { SyncService } from '../sync/sync.service';
 import { AuthenticatedUser } from './authenticated-request';
+import { toAuthenticatedUser } from './provision-user';
 
 export interface TeamJwtPayload {
   sub: string;
@@ -146,15 +147,8 @@ export class TeamAuthService {
     if (!user) {
       throw new UnauthorizedException('Team account no longer exists');
     }
-    return {
-      userId: user.id,
-      authentikSub: user.authentikSub,
-      email: user.email,
-      memberships: user.memberships.map((m) => ({
-        kcId: m.kcId,
-        gemeindeId: m.gemeindeId,
-        role: m.role,
-      })),
-    };
+    // Same shape as the Authentik path, incl. the synthetic global
+    // LEITUNGSTEAM membership when `isLeitungsteam` is set on the row.
+    return toAuthenticatedUser(user);
   }
 }