Defer ambiguous cross-account Email/copy retries

Problem

If Stalwart commits a cross-account Email/copy but the response is lost, Stormbox may retry and create a duplicate. JMAP has no persistent copy idempotency key, and Stalwart currently creates a new Email instead of returning alreadyExists with the first copy's ID.

The same upstream behavior is needed when Stormbox receives a successful copy response but destination cache/view reconciliation subsequently fails. Retrying the whole copy is only safe if Stalwart returns alreadyExists(existingId); otherwise a retry can duplicate the already-confirmed copy. We are intentionally not adding a separate server_response_json reconciliation checkpoint while waiting for the server fix.

Decision

Do not add a client-side marker or checkpoint workaround for now. Wait for Stalwart to implement Cyrus-style duplicate detection for repeated Email/copy requests. Once available, Stormbox must fetch existingId, ensure the requested destination mailbox membership, and then reconcile the destination cache.

This affects only cross-account copies; same-account moves use Email/set. Exposure is currently limited because shared/multi-account workflows are not widely used.

Related: #79

Start by tracking Stalwart's duplicate-detection fix for repeated cross-account Email/copy requests; no Stormbox file or test is named here. Once alreadyExists(existingId) is available, follow the stated flow: fetch existingId, ensure destination mailbox membership, and reconcile the destination cache without adding a client-side marker or checkpoint.