Add Authentik SSO via OAuth2/OIDC

Implements Single Sign-On with Authentik alongside the existing
password login. Admins configure Client ID, Client Secret and Base URL
directly in the app; the SSO button on the login page only appears
when the configuration is complete and enabled.

- SsoSetting model + migration (one-row config table)
- sso_provider_id / is_sso_user fields on users (migration)
- SsoController: redirect to Authentik + callback (token exchange,
  userinfo fetch, auto-create unknown users)
- Admin\SsoSettingController + admin/sso/show view with setup guide
  and one-click Redirect URI copy
- Admin dropdown: SSO-Konfiguration entry
- Login page: Authentik button rendered conditionally

No additional Composer packages required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

resources/views/auth/login.blade.php

@@ -74,6 +74,21 @@
                     </button>
                 </form>
 
+                @php $sso = \App\Models\SsoSetting::get(); @endphp
+                @if($sso->isConfigured())
+                    <div class="d-flex align-items-center my-4" style="gap:12px;">
+                        <hr style="flex:1; border-color:rgba(0,0,0,.12); margin:0;">
+                        <span style="font-size:.78rem; color:#9ca3af; white-space:nowrap;">oder</span>
+                        <hr style="flex:1; border-color:rgba(0,0,0,.12); margin:0;">
+                    </div>
+                    <a href="{{ route('sso.redirect') }}" class="btn btn-outline-secondary w-100 btn-lg d-flex align-items-center justify-content-center gap-2">
+                        <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+                            <circle cx="12" cy="12" r="10"/><path d="M12 2a14.5 14.5 0 0 0 0 20A14.5 14.5 0 0 0 12 2"/><path d="M2 12h20"/>
+                        </svg>
+                        Mit Authentik anmelden
+                    </a>
+                @endif
+
             </div>
         </div>