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/management/commands/init_config.py

@@ -91,10 +91,76 @@ class Command(BaseCommand):
             },
         ]
 
+        # E-Mail / IMAP Settings
+        email_settings = [
+            {
+                "key": "imap_host",
+                "display_name": "IMAP Server",
+                "description": "Hostname oder IP-Adresse des IMAP-Servers (z.B. mail.example.com)",
+                "value": "",
+                "default_value": "",
+                "setting_type": "text",
+                "category": "email",
+                "order": 1,
+            },
+            {
+                "key": "imap_port",
+                "display_name": "IMAP Port",
+                "description": "Port des IMAP-Servers (Standard: 993 für SSL, 143 für unverschlüsselt)",
+                "value": "993",
+                "default_value": "993",
+                "setting_type": "number",
+                "category": "email",
+                "order": 2,
+            },
+            {
+                "key": "imap_user",
+                "display_name": "IMAP Benutzername",
+                "description": "Benutzername / E-Mail-Adresse für die IMAP-Anmeldung",
+                "value": "",
+                "default_value": "",
+                "setting_type": "text",
+                "category": "email",
+                "order": 3,
+            },
+            {
+                "key": "imap_password",
+                "display_name": "IMAP Passwort",
+                "description": "Passwort für die IMAP-Anmeldung",
+                "value": "",
+                "default_value": "",
+                "setting_type": "password",
+                "category": "email",
+                "order": 4,
+            },
+            {
+                "key": "imap_folder",
+                "display_name": "IMAP Ordner",
+                "description": "Name des zu überwachenden Postfach-Ordners (Standard: INBOX)",
+                "value": "INBOX",
+                "default_value": "INBOX",
+                "setting_type": "text",
+                "category": "email",
+                "order": 5,
+            },
+            {
+                "key": "imap_use_ssl",
+                "display_name": "SSL/TLS verwenden",
+                "description": "Sichere Verbindung zum IMAP-Server (empfohlen)",
+                "value": "True",
+                "default_value": "True",
+                "setting_type": "boolean",
+                "category": "email",
+                "order": 6,
+            },
+        ]
+
+        all_settings = paperless_settings + email_settings
+
         created_count = 0
         updated_count = 0
 
-        for setting_data in paperless_settings:
+        for setting_data in all_settings:
             setting, created = AppConfiguration.objects.get_or_create(
                 key=setting_data["key"], defaults=setting_data
             )