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/stiftung/models/system.py

@@ -313,6 +313,7 @@ class AppConfiguration(models.Model):
 
     SETTING_TYPE_CHOICES = [
         ("text", "Text"),
+        ("password", "Password"),
         ("number", "Number"),
         ("boolean", "Boolean"),
         ("url", "URL"),
@@ -322,6 +323,7 @@ class AppConfiguration(models.Model):
 
     CATEGORY_CHOICES = [
         ("paperless", "Paperless Integration"),
+        ("email", "E-Mail / IMAP"),
         ("general", "General Settings"),
         ("corporate", "Corporate Identity"),
         ("notifications", "Notifications"),