Fix single checkin GET to include username via JOIN

server.py

@@ -1584,7 +1584,7 @@ def list_checkins(
 @app.get("/api/checkins/{checkin_id}")
 def get_checkin(checkin_id: int):
     conn = get_db()
-    row = conn.execute("SELECT * FROM checkins WHERE id = ?", (checkin_id,)).fetchone()
+    row = conn.execute("SELECT c.*, u.username FROM checkins c LEFT JOIN users u ON c.user_id = u.id WHERE c.id = ?", (checkin_id,)).fetchone()
     if row is None:
         conn.close()
         raise HTTPException(status_code=404, detail="Checkin not found")