Integrate MongoDB for full order_spec persist order_spec JSON to MongoDB (Adapter pattern)

[raggopaluni]

Goal:

• Save the full JSON order_spec (the original request body) into MongoDB, associated with order_id.

Tasks:

• Add services/mongo_client.py with Mongo client (Singleton pattern).
• Create services/repository.py with an OrderRepository interface and adapters:
  • PostgresRepository (for metadata)
  • MongoRepository (for order_spec)
• On POST /orders: insert doc into Mongo: {order_id, spec, created_at}
• Modify GET /orders/{order_id} to combine Postgres metadata + Mongo order_spec in response.

Acceptance criteria:

• Creating an order persists data in both Postgres and Mongo.
• GET returns combined view: metadata + spec.