Add IMAP configuration UI and sidebar navigation for email inbox

- Email settings page at /administration/email/ with IMAP config form
- Connection test button to verify IMAP connectivity
- Sidebar link "E-Mail Eingang" for quick access
- AppConfiguration model extended with email category and password type
- init_config command includes IMAP default settings
- DB-based IMAP config with env var fallback

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

app/templates/stiftung/administration.html

@@ -217,6 +217,12 @@
                                 <span>App Settings</span>
                             </a>
                         </div>
+                        <div class="col-md-3 mb-3">
+                            <a href="{% url 'stiftung:email_settings' %}" class="btn btn-outline-primary w-100">
+                                <i class="fas fa-envelope d-block mb-2 fa-2x"></i>
+                                <span>E-Mail / IMAP</span>
+                            </a>
+                        </div>
                         <div class="col-md-3 mb-3">
                             <a href="{% url 'stiftung:audit_log_list' %}" class="btn btn-outline-primary w-100">
                                 <i class="fas fa-history d-block mb-2 fa-2x"></i>