feat(api): add idempotency key support for mutating operations

[martsokha]

Context

File uploads, job dispatch, and webhook delivery lack idempotency support. This is important for retry safety in distributed systems.

Requirements

• Accept optional Idempotency-Key header on mutating operations
• Deduplicate requests with the same key within a configurable window
• Return the original response for duplicate requests
• Apply to: file upload, job dispatch, webhook delivery, and other POST endpoints
• Store idempotency state with TTL (in-memory or database-backed)

References

• Server docs: ARCHITECTURE.md - API Design