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/base.html

@@ -596,6 +596,10 @@
                 <i class="fas fa-gift"></i>
                 <span>Foerderungen</span>
             </a>
+            <a class="sidebar-link" href="{% url 'stiftung:email_eingang_list' %}">
+                <i class="fas fa-envelope-open-text"></i>
+                <span>E-Mail Eingang</span>
+            </a>
             <a class="sidebar-link" href="{% url 'stiftung:paechter_list' %}">
                 <i class="fas fa-user-tie"></i>
                 <span>Paechter</span>