Send Idempotency-Key on POST/PUT/PATCH so retries are safe

[justin-layerv]

Summary

After the next qURL prod release, the API will support the `Idempotency-Key` header end-to-end on state-mutating endpoints — so a retried request with the same key returns the original response instead of creating a duplicate.

This SDK should generate and forward that header automatically so users do not have to think about it.

Why

`#19` ("POST retries on network errors can duplicate items (batch_create especially)") is the user-facing symptom. Today, a transient 5xx or socket reset on `client.create()` / `batch_create()` / mint flows can produce duplicate resources after the SDK retries — because each retry is a fresh request from the server's perspective. With idempotency-key forwarding in place, the server collapses retries onto the original response.

Proposed surface

• Auto-generate a UUIDv7 `Idempotency-Key` per outbound POST/PUT/PATCH (read-only methods do not need it).
• Reuse the same key across retries of the same logical call — do NOT regenerate inside the retry loop, otherwise the server sees N independent writes.
• User override via a per-call option (e.g. `idempotency_key=...` on the public methods) for callers that want to tie idempotency to their own request ID (e.g. an upstream job ID) instead of an SDK-generated UUID.
• Apply to both the sync (`client.py`) and async (`async_client.py`) clients.
• Documentation: brief README section explaining what the SDK now does on retries, and how to override; docstrings on the affected methods.

Definition of done

• UUIDv7 generated per POST/PUT/PATCH and sent as `Idempotency-Key`.
• Retry path reuses the same key.
• Per-call override exposed (sync + async).
• README/docstrings updated.
• Test: simulate a transient 5xx + retry, assert the second request carries the same `Idempotency-Key` value as the first (and that the SDK does not surface a duplicate on the caller side when the server returns the cached response).
• Closes POST retries on network errors can duplicate items (batch_create especially) #19.

Sequencing

Gate landing this on the qURL prod release that ships server-side `Idempotency-Key` support on `POST /v1/api-keys`. The generic middleware on other POST endpoints is already live, so the SDK can begin sending the header sooner — but holding until the API-key path is also covered keeps the rollout coherent.