Fix events: return 404 for deleted/missing calendar entries

The before_request handler loaded the entry but didn't abort when it was
None, causing template UndefinedError when building URLs with entry.id.

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

events/bp/calendar_entry/routes.py

@@ -50,6 +50,7 @@ def register():
     @bp.before_request
     async def load_entry():
         """Load the calendar entry from the URL parameter."""
+        from quart import abort
         entry_id = request.view_args.get("entry_id")
         if entry_id:
             result = await g.s.execute(
@@ -60,6 +61,8 @@ def register():
                 )
             )
             g.entry = result.scalar_one_or_none()
+            if g.entry is None:
+                abort(404)
 
     @bp.context_processor
     async def inject_entry():