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/restoretest_due.go

@@ -140,3 +140,29 @@ func (s *Scheduler) evaluateTier(ctx context.Context, target string, cutoff time
 func (s *Scheduler) EvaluateDueTier(ctx context.Context, target string) DueVerdict {
 	return s.evaluateTier(ctx, target, s.settleCutoff())
 }
+
+// verdictSummary renders one compact line of per-tier verdicts for the "nothing due" log.
+//
+// It re-evaluates rather than threading the verdicts out of pickForThisRun, and that is a
+// deliberate trade: this runs only on the path where NOTHING is due, so the cost is one extra
+// storage listing per tier on an otherwise idle evaluation (measured 18 ms local / 392 ms offsite,
+// R-86 Part 1.4), and in exchange the logging path cannot drift from the deciding path by holding a
+// stale copy of it. If that cost ever matters, pass the verdicts in — do not let the two diverge.
+func (s *Scheduler) verdictSummary(ctx context.Context) string {
+	out := ""
+	for _, v := range s.EvaluateDue(ctx) {
+		if out != "" {
+			out += "; "
+		}
+		switch {
+		case v.Err != nil:
+			out += v.Target + ": UNKNOWN (" + v.Err.Error() + ")"
+		default:
+			out += v.Target + ": " + v.Reason
+		}
+	}
+	if out == "" {
+		return "no tiers configured"
+	}
+	return out
+}