perf: skip the WAL archive destination check while archiving succeeds

[melancholictheory]

Fixes #156.

What

The instance sidecar remembers the destination of the last WAL batch it archived successfully (the Archive UID and generation, plus the stanza) and skips CheckWalArchiveDestination while batches keep succeeding against it. That check runs pgbackrest info against the repository, which on S3 is six requests per batch on top of the three archive-push needs.

A failed batch or any change to the Archive spec brings the check back. The first batch after the sidecar starts still runs it, so the stanza creation from #121 works as before. The restore job leaves DestinationCheck nil and keeps checking before every batch.

How

• internal/cnpgi/common/destination.go adds DestinationCheck, which keeps the key of the last verified destination behind a mutex. A nil *DestinationCheck remembers nothing.
• Archive in wal.go moves the existing check, unchanged, into checkDestination and runs it only when the key is not verified. A failed batch resets the key and a successful one records it.
• instance/start.go gives the instance WAL service its DestinationCheck.

There is one difference in behaviour. If the stanza is deleted from the repository while archiving runs, the next archive-push fails once (exit 103) before the check runs again and recreates the stanza. PostgreSQL retries that segment.

Testing

• Unit tests for DestinationCheck, and go test -race ./internal/cnpgi/... passes.
• golangci-lint v2.12.2, the version CI uses, reports no issues.
• CI passes on our fork, e2e suite included (7 of 7), against CloudNativePG 1.30.0 with the MinIO images from fix(e2e): run MinIO from the Chainguard images #155. Against CloudNativePG main the restore specs hang until feat: serve restore hooks from the instance sidecar #154 is merged, because feat(bootstrap): use an init container instead of a separate Job for new instances cloudnative-pg/cloudnative-pg#11319 moved the restore into the bootstrap-instance init container. That is unrelated to this change, but CI here will show it.
• With the change MinIO logs three requests per segment instead of nine. The throughput comparison is in WAL archiving runs pgbackrest info before every batch #156.

[melancholictheory]

CI is red only on the e2e suite, for a reason unrelated to this change: docker.io/minio/minio:latest no longer exists on Docker Hub (pull access denied, repository does not exist), so the MinIO pods never start and the specs time out. Spellcheck, commitlint, lint, the unit tests and the uncommitted check pass.

#154 includes the switch to the Chainguard MinIO images from #155 and makes the restore specs pass on CloudNativePG main. Merged on top of #154, this branch passes the whole e2e suite against CloudNativePG main on our fork, 8 of 8 (#154 adds one spec): run. We will rebase once #154 is merged.