feat: Implement upcoming appointments feature for users

- Added `remove_done` function to clean up finished appointments in the database.
- Introduced `get_upcoming_for_user` function to retrieve upcoming appointments for a specific user.
- Updated `backend_server.py` to include a new endpoint for fetching user-specific upcoming events.
- Modified `blueprint.py` to render upcoming events in the user interface.
- Enhanced `terminplan.html` to display a list of upcoming appointments with relevant details and action links.
- Updated configuration to enable the new appointment planning feature.

config.json

@@ -53,6 +53,9 @@
         "inventory": {
             "enabled": true
         },
+        "terminplan": {
+            "enabled": true
+        },
         "library": {
             "enabled": true
         },