A cadence cap on the local-API whole-guest backup route (R-973, unreleased)

Each successful run prunes its tier, so a compromised controller calling POST /backup in a loop
pushed out every pre-compromise restore point. A tier with a cadence of at most 24 h now allows 3
successful runs per rolling 24 h (night, a press, the next night); a longer tier 1 per
max(cadence - 24 h, cadence/2), 6 days for the weekly off-site tier. Only successes count. A
refusal is a 429, a WARN, saved, and for 24 h a critical host-report status that the hub turns
into its existing operator-only agent_capability_degraded event. The success history and the
refusal are kept in the R-894 state file, so a restart does not reset the cap. No sudoers change.
Tests TestBackupCap_* and the state, status and wiring tests, red-proved.

internal/localapi/server.go

@@ -90,6 +90,10 @@ type BackupStore interface {
 type LastKnownBackupStore interface {
 	RecordBackupSuccess(target string, b hub.Backup) error
 	LastKnownSuccess(target string, vmid int) (time.Time, bool)
+	// R-973: the cadence cap's restart-proof history and its refusals.
+	RecentBackupSuccesses(target string, vmid int, since time.Time) []time.Time
+	RecordBackupRefusal(target string, vmid int, at time.Time) error
+	LastBackupRefusal() (target string, vmid int, at time.Time, ok bool)
 }
 
 // StorageView yields the host's observed storage targets (for mapping a mount's storage id →
@@ -302,6 +306,11 @@ type Server struct {
 	logger             *slog.Logger
 	now                func() time.Time
 
+	// R-973 cadence cap: this process's successful runs per tier+guest, and the newest refusal.
+	capMu      sync.Mutex
+	capRuns    map[backupJobKey][]time.Time
+	capRefusal capRefusal
+
 	disks           DiskOps           // slice 8C (optional)
 	diskGate        StorageGate       // slice 8C (optional)
 	guestList       GuestLister       // slice 8C (optional)
@@ -880,6 +889,12 @@ func (s *Server) handleBackup(w http.ResponseWriter, r *http.Request, vmid int)
 			"a backup is already in flight on target "+busyTarget+" (job "+busyJob+") — only one backup runs at a time per guest")
 		return
 	}
+	// R-973: the cadence cap (backup_cap.go). Checked under jobsMu, after the in-flight answers, so two
+	// requests cannot both pass it (only one run per guest is ever in flight).
+	if s.capRefuses(w, key, tier) {
+		s.jobsMu.Unlock()
+		return
+	}
 	// Job ids must be unique PER TIER, and by construction rather than by clock luck: two tiers
 	// started inside the same nanosecond (the weekly both-due night, or any injected clock) would
 	// otherwise collide and hand the second caller the first tier's id. The PRIMARY keeps the
@@ -943,6 +958,9 @@ func (s *Server) handleBackup(w http.ResponseWriter, r *http.Request, vmid int)
 			s.logger.Info("local-api: backup job complete", "vmid", vmid, "target", tier.TargetID, "job", jobID, "archive", b.Archive)
 		}
 		s.store.RecordBackup(b)
+		if b.Success {
+			s.recordCapRun(key, b) // R-973
+		}
 		if b.Success && s.lastKnown != nil {
 			if err := s.lastKnown.RecordBackupSuccess(tier.TargetID, b); err != nil {
 				// Not fatal: the backup exists. Only the fallback after a restart loses this copy.