Add Redis caching for GET endpoint - add Redis caching for order reads (cache order summary)

[raggopaluni]

Goal:

• Use Redis to cache order summary (metadata + small spec fields) to speed up reads.

Tasks:

• Add services/redis_client.py (Singleton).
• On GET /orders/{order_id}:
  • Check Redis cached key order:{order_id}.
  • If hit, return cached value.
  • If miss, fetch from Postgres+Mongo, cache a summary with TTL 300s, then return.
• Add cache invalidation on PUT/DELETE if implemented.
• Add Redis to docker-compose.

Acceptance criteria:

• Repeated GETs within TTL show cache hits (log or metric).
• Cache is invalidated/updated upon order updates (if PUT implemented).