Add MongoDB Node Driver + DocumentDB Playground

[hossain-rayhan]

Summary

• Add a DocumentDB playground using the native MongoDB Node.js driver.
• Provide an Express REST API with CRUD and genre aggregation endpoints.
• Add compatibility coverage for CRUD, indexes, aggregation, unique constraints, and vector search.
• Add scripts for DocumentDB lifecycle, app startup, and test execution.
• Document setup, configuration, API usage, and compatibility notes.
• Update the root README with the new playground.

Verification

• ./scripts/run-test.sh: 16 passed, 0 failed
• PORT=3101 ./scripts/run-app.sh: started successfully
• /health: returned {"status":"healthy","db":"connected"}
• Verified create, list, update, aggregation, and delete API flows
• JavaScript and shell syntax checks passed