v0.121.1: 'nothing is due' must be AUDIBLE (R-86 + standing rule 3)

Before R-86 every tick ran a heavy restore-test, so the scheduler was audible by
construction. After it, 'nothing is due' is the NORMAL outcome — and it was
logged at DEBUG, which journald drops. An empty journal would then be equally
consistent with a healthy loop and a dead goroutine: the shape the R-88 watcher
was retired for, re-created by making the quiet path the common one.

A not-due evaluation now logs one INFO line naming every tier's verdict (four
lines a day at the 6h default), and an unlistable tier reads UNKNOWN with its
error in that same line, so a lookup failure can never present as 'nothing due'.

Red-proved through the scheduler's own tick, not the helper.

internal/backup/schedule.go

@@ -180,7 +180,15 @@ func (s *Scheduler) tick(ctx context.Context) {
 		return
 	}
 	if archive == "" {
-		s.logger.Debug("backup: restore-test not due this evaluation")
+		// A POSITIVE OBSERVABLE, at INFO, and this is not noise — it is standing rule 3.
+		//
+		// Before R-86 every tick ran a heavy restore-test, so the scheduler was audible by
+		// construction. Now "nothing is due" is the NORMAL outcome, and at DEBUG it is silent: an
+		// empty journal would be equally consistent with a healthy loop and with a dead goroutine,
+		// which is the exact shape the R-88 watcher was retired for. One line per evaluation is four
+		// lines a day at the 6h default, and it names each tier's verdict so the answer to "why did
+		// nothing run last night?" is in the log rather than in a re-derivation.
+		s.logger.Info("backup: restore-test evaluated — nothing due", "verdicts", s.verdictSummary(ctx))
 		return
 	}