R-86: restore-test follows the backup, not the clock (v0.121.0)

The ticker survives as the EVALUATION interval only. A tier is DUE when its
newest archive that has settled for `settle` (default 24h) has not been proven:
daily tier -> proved daily on yesterday's archive, weekly tier -> weekly on its
own, newborn -> UNKNOWN.

The trap avoided: the literal reading ("newest archive is >= 24h old") is NEVER
true on a daily tier, so it silently switches restore-testing off where it
matters most. Red-proved at 0 runs over 5 simulated days.

- state records WHICH archive was proven; legacy files keep their time and yield
  no proven archive (each tier due once after the upgrade, deliberately)
- two knobs replace one: restore_test_eval_interval_seconds (6h, measured) and
  restore_test_settle_seconds (24h). The old cadence key keeps its DISABLE
  meaning verbatim and now seeds the settle lag, with a start-up WARN.
- due-check runs BEFORE the heavy-op gate (a frequent poll must not make a
  starting backup record a failure, F-A1)
- candidate picker skips implausible archives (a phantom would be due forever)
- new read-only --selftest=restore-test-due prints the verdict + its cost

internal/backup/backup_test.go

@@ -135,10 +135,11 @@ func TestBackup_VzdumpFailureReturnsFailedRecord(t *testing.T) {
 }
 
 func TestPickRestoreCandidate_NewestOrEmpty(t *testing.T) {
+	const big = 4 << 30 // a plausible whole-guest archive
 	api := &fakeBackupAPI{content: []proxmox.StorageContent{
-		{VolID: "a", Content: "backup", CTime: 10},
-		{VolID: "b", Content: "backup", CTime: 99},
-		{VolID: "iso", Content: "iso", CTime: 999}, // not a backup → ignored
+		{VolID: "a", Content: "backup", CTime: 10, Size: big},
+		{VolID: "b", Content: "backup", CTime: 99, Size: big},
+		{VolID: "iso", Content: "iso", CTime: 999, Size: big}, // not a backup → ignored
 	}}
 	r := NewBackupRunner(api, "local", "", "", "", quiet())
 	vol, err := r.PickRestoreCandidate(context.Background())
@@ -152,6 +153,26 @@ func TestPickRestoreCandidate_NewestOrEmpty(t *testing.T) {
 	}
 }
 
+// R-86: the NEWEST entry is not a candidate if it cannot be a complete archive. An incomplete
+// artefact (F-CRIT-2's 1-byte phantom, which server-side prune does not collect) would otherwise be
+// picked forever, fail its restore forever, never earn proof, and so leave the tier due at every
+// evaluation — turning the evaluation interval into the retry rate for a multi-GB restore.
+//
+// COMPANION RED-PROOF (observed): drop the `archivePlausiblyComplete` guard from
+// PickSettledRestoreCandidateOn and this fails with
+// `pick = "phantom" want the newest COMPLETE archive 'real'`.
+func TestPickRestoreCandidate_SkipsImplausibleArchives(t *testing.T) {
+	api := &fakeBackupAPI{content: []proxmox.StorageContent{
+		{VolID: "real", Content: "backup", CTime: 10, Size: 4 << 30},
+		{VolID: "phantom", Content: "backup", CTime: 99, Size: 1}, // newest, and impossible
+	}}
+	r := NewBackupRunner(api, "local", "", "", "", quiet())
+	vol, err := r.PickRestoreCandidate(context.Background())
+	if err != nil || vol != "real" {
+		t.Fatalf("pick = %q,%v want the newest COMPLETE archive 'real'", vol, err)
+	}
+}
+
 // --- scheduler ---
 
 type fakeRTRunner struct {