Backups: collection interval is actually readable

CHANGELOG.md

@@ -32,6 +32,11 @@ ADR-007 and ADR-002.
   PC's revisions can no longer push out a quiet PC's only backup, and the
   revision diff compares against the same PC's previous revision rather than
   another PC's.
+- The backup collection interval could not actually be changed. The collecting
+  script reads `backups_intervalhours` from the public settings endpoint before
+  it holds any credential, but the plugin did not declare the key as public, so
+  the endpoint never returned it and the script fell back to its built-in 24
+  hours - silently. The setting looked configurable in the UI and was not.
 - A backup export containing a value with an empty right-hand side (`Name=`)
   failed to parse, and with it the whole file, so that machine could never be
   backed up. The form is not strictly legal but occurs in real exports. It is