Integrate Postgres (SQLAlchemy) for order metadata (HIGH) add Postgres + SQLAlchemy to store order metadata

[raggopaluni]

Goal:

• Persist order metadata (order_id UUID, status, created_at) in Postgres using SQLAlchemy.

Tasks:

• Add services/pg_client.py with SQLAlchemy engine + session factory (Singleton pattern).
• Add services/models.py with OrderMeta SQLAlchemy model.
• On POST /orders: generate UUID, insert OrderMeta row (status="created"), return order_id.
• Add GET /orders/{order_id} to fetch metadata from Postgres.
• Provide local Postgres Docker service in docker-compose.yml for dev.
• Add migration notes (no Alembic required initially; use create_all()).

Acceptance criteria:

• Postgres container spins up via docker-compose.
• Creating an order writes to Postgres and GET returns saved metadata.