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>

client/app/lib/screens/home_screen.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../api.dart';
 import '../main.dart';
+import 'admin_screen.dart';
 import 'chat_screen.dart';
 import 'files_screen.dart';
 import 'wahl_screen.dart';
@@ -16,6 +17,13 @@ class HomeScreen extends StatelessWidget {
     final kcId = id.kcId;
 
     final tiles = <Widget>[
+      if (id.isLeitungsteam)
+        _NavTile(
+          icon: Icons.admin_panel_settings,
+          title: 'Verwaltung',
+          subtitle: 'KCs, Gemeinden, Onboarding-Freigaben',
+          onTap: () => _open(context, const AdminScreen()),
+        ),
       if (id.kind == SessionKind.guest)
         _NavTile(
           icon: Icons.how_to_vote,