Add index creation step to CI for aggregation performance

Create index on comments.movie_id after data restore to fix the
/reportingByComments aggregation timeout. Without this index, the
$lookup operation performs a full collection scan (41K comments x 21K
movies) causing 5+ minute response times in CI vs ~700ms locally.

Author: cbullinger

.github/workflows/run-express-tests.yml

@@ -41,6 +41,10 @@ jobs:
       - name: Add sample data to database
         run: mongorestore --archive=sampledata.archive --port=27017
 
+      - name: Create indexes for aggregation performance
+        run: |
+          mongosh "mongodb://localhost:27017/sample_mflix?directConnection=true" --eval "db.comments.createIndex({ movie_id: 1 })"
+
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with: