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/views/__init__.py

@@ -156,6 +156,7 @@ from .system import (  # noqa: F401
     user_login,
     user_logout,
     app_settings,
+    email_settings,
     edit_help_box,
     two_factor_setup,
     two_factor_qr,