fix(backup): ignore NotFound after deletion - #11484

❗ By default, the pull request is configured to backport to all release branches.

• To stop backporting this pr, remove the label: backport-requested ◀️ or add the label 'do not backport'
• To stop backporting this pr to a certain release branch, remove the specific branch label: release-x.y

The fix itself matches the convention the other reconcilers already use for a deleted object: DatabaseRoleReconciler (internal/controller/databaserole_controller.go:71-74), PoolerReconciler (pooler_controller.go:76-79), and ScheduledBackupReconciler (scheduledbackup_controller.go:85-87) all return ctrl.Result{}, nil on IsNotFound, logging "Resource has been deleted" rather than wrapping it in reconcile.TerminalError. BackupReconciler was the outlier before this PR.

The two red checks aren't about the fix, they're both mechanical and easy to clear:

"Ensure Pull Request has a linked issue" is failing because of the URL, not because the issue isn't referenced. The checker (.github/workflows/pr_verify_linked_issue.yml:49-58) only matches a keyword immediately followed by #<number> (optionally owner/repo#<number>) — a full https://.../issues/11483 URL after the keyword doesn't match its regex. I ran the exact pattern from that file against the current body:

input:  "Resolves: https://github.com/cloudnative-pg/cloudnative-pg/issues/11483"
matches: []

input:  "Resolves #11483"
matches: ["Resolves #11483"]

Swapping the body to Resolves #11483 (colon optional either way) is enough to pass.

DCO is failing because the commit has no Signed-off-by: trailer — git show 79c4da9 is just the subject line, nothing else. git commit --amend -s (or an interactive rebase with -s if there end up being more commits) then a force-push should clear it.

Nothing else jumped out from a read of the diff and the added test.