Module idempotency

Expand description

Idempotency-Key layer — safe client retries for mutating endpoints.

Stripe-style semantics for #[Idempotent(ttl = "24h")] routes:

Client sends Idempotency-Key: <opaque>; the key is scoped to tenant + principal + route, so keys can never collide across users.
First arrival claims the key atomically and runs the handler; the final response (status < 500) is stored for ttl.
A retry with the same key gets the stored response replayed (Idempotency-Replayed: true) — no second order, no double charge.
A concurrent duplicate while the first is still running gets 409 Conflict (retry after the original completes).
Handler failures ≥ 500 release the claim so the client may retry.
Requests without the header pass through untouched.

Pairs deliberately with #[Timeout]: deadlines make clients retry; this layer makes those retries safe.

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