tests: implement concurrency and load tests for inventory and orders

[codewithme-py]

• feat(tests): add pytest concurrency tests for inventory reservations and ARQ expiry worker using asyncio.gather
• feat(load): implement Locust scenarios (HighLoadUser, OversellTestUser, OrderLoadUser) to stress test reserve and order creation flows
• fix(arq): resolve race condition causing double inventory returns on simultaneous reservation expirations
• fix(db): increase SQLAlchemy QueuePool size (POOL_SIZE=200) to handle high concurrency locking (SELECT FOR UPDATE) under load
• fix(api): fix TypeError in @idempotent decorator when serializing SQLAlchemy models (Order) by using jsonable_encoder
• fix(api): add explicit eager loading for Order.items relationship to resolve FastAPI ResponseValidationError
• fix(nginx): configure limit_req_status to return 429 Too Many Requests instead of 503 during extreme load bursts
• chore: refactor Locust load test files into dedicated load_tests/ directory and update Makefile test targets