Feature — Idempotency Key Support

[coderkrp]

As a developer,
I want to provide an idempotency key for orders,
So that retries are safe and don’t create duplicates.

Acceptance criteria

• AC1: The order placement endpoint (/api/v1/orders) accepts an Idempotency-Key header.
• AC2: The orchestrator checks for the existence of this key in a persistent store (SQLite/Postgres).
• AC3: If the key is found, the orchestrator immediately returns the cached response from the original request without calling the adapters.
• AC4: If the key is not found, the orchestrator proceeds with the request and stores the key and the final UnifiedResponse upon completion.
• AC5: The stored keys have a configurable TTL (e.g., 24 hours).

Definition of Done

• Code implemented in a feature/fix branch
• Unit / integration tests added or existing tests updated
• Linting passed and code builds locally
• Docs / README updated if behavior changed
• PR opened and linked (Closes #<issue-number>)
• Manual QA steps verified (if applicable)