fix(gateway): register the shared ACME account under the rotation lock

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - certbot: a certificate covering both a name and its wildcard (`example.com` and `*.example.com`) could never be issued over dns-01. The two authorizations are answered under one `_acme-challenge.example.com`, each with its own TXT value, and the publish step cleared every TXT record at that name before writing its own -- so the second authorization deleted the record answering the first, and the order failed with `Correct value not found for DNS challenge`. Clearing leftovers from an aborted run is now done once per challenge name per issuance, and the records for one name accumulate instead of replacing each other; cleanup afterwards is unchanged, deleting each record this run created by id
 - certbot: editing `domains` in `certbot.toml` had no effect once a certificate existed. Issuance was skipped whenever `live/cert.pem` was present, whatever names it carried, and renewal read its name list back off that certificate rather than the configuration -- so an added or removed name never reached the CA, and the mismatch survived every renewal. The live certificate's DNS names are now compared against the configured list (as sets, case- and trailing-dot-insensitive) and a mismatch reissues, logging both lists. A reissue that fails does not take the renewal check down with it: a name the CA will not validate is reported on every cycle, while the certificate actually being served keeps renewing, and the failure is still what the run returns unless the renewal committed something of its own
 - certbot: `certbot cfg` attached every comment after `cf_api_url` to the wrong key, because an absent optional field shifts the generated document's keys out of step with the struct's. The template's documentation is now looked up by key name
+- gateway: a fresh cluster could register several ACME accounts at once. The shared account was registered lazily, on whichever renewal first found the credentials record empty, under no lock but the per-domain renewal lock -- so two domains, or two nodes, starting together each spent a rate-limited registration, and the last-writer-wins credentials record kept exactly one of them. The attestation written beside it races under its own key, so the account the cluster ends up using need not be the one it can prove it holds. One lock in the KV store now covers every operation over the shared account -- rotation, CAA reconciliation, and first-use registration -- so they are ordered across nodes and not merely within one process, as CAA reconciliation was before. Registration re-reads the record under that lock and adopts an account another node registered while it waited, and builds the DNS provider client only after the lock is granted, so a refused attempt costs no provider API call
 - gateway: a node removed via `RemoveNode` silently rejoined the cluster the next time it started, because every node re-registers its own sync address on boot. Once tombstone GC is collecting, that comeback is worse than an annoyance: a stale data directory diverges from every digest, and the divergence repair's full re-exchange resurrects records whose deletes the cluster already collected. Removal now writes a durable marker — a live record, so the GC can never eat it — that every gateway's sync endpoints enforce; a removed node's envelopes are refused until an operator re-admits it with `SetNodeUrl`. The refused node counts HTTP 403 sync rejections (`dstack_gateway_sync_rejected_total`), including removal lockouts and app-identity mismatches. Every gateway also exposes `dstack_gateway_node_last_seen_timestamp_seconds` per known node, so a long-offline gateway is a one-line alert instead of an ack-watermark puzzle
 - gateway: deleted KV records left a tombstone that nothing ever collected, so every deregistered CVM stayed on disk for the life of the deployment. Tombstones every peer has acknowledged are now dropped once every `tombstone_gc_writes` replicated writes (default 10000, zero disables); the trigger counts replicated writes rather than reading a clock, so nodes in a cluster collect in the same window without depending on time synchronization. A `SetTombstoneGcConfig` admin RPC stores an operator override in the KV itself, replicating one pace to every node
 - gateway: `Admin.RemoveCvm` now reports the outcome of the `inst/` tombstone alone. A failure to delete associated override or telemetry records is logged instead of failing the call, so a removal that did take effect is no longer reported as failed — which also aborted the local routing cleanup that follows it. Re-issuing a removal still sweeps up override and telemetry records orphaned by an earlier partial failure