feat: add Nextcloud config placeholder and Docker docs

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

DOCKER_CONFIG.md

@@ -62,3 +62,17 @@ No migration needed - existing installations will automatically:
 ## File Format
 
 Configuration files use standard JSON format and can be edited directly if needed. Changes take effect after application restart.
+
+## Nextcloud Calendar Sync
+
+To enable Nextcloud CalDAV sync, populate the `Nextcloud` section in the production `appsettings.json` (not committed to git):
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "your-nextcloud-username",
+  "AppPassword": "your-nextcloud-app-password"
+}
+```
+
+Generate an app password in Nextcloud: **Settings → Security → Devices & sessions → Create new app password**.

Server/appsettings.json

@@ -18,6 +18,12 @@
     ]
   },
 
+  "Nextcloud": {
+    "BaseUrl": "https://your-nextcloud-instance.example.com",
+    "Username": "YOUR_NEXTCLOUD_USERNAME",
+    "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+  },
+
   "Transport": {
     "StartStation": "Zurich",
     "EndStation": "Basel",