server: enqueue pending_runs when scheduled-job dispatch fails

When dispatchBackupForGroup's conn.Send errors, queue a pending_runs
row (attempt=1, next_attempt_at = now + group.RetryBackoffSeconds)
instead of silently dropping the fire. The orphaned queued job row
is left behind for forensic visibility — the drainer will create a
fresh job row on its retry.

Also adds Store.ListPendingRunsForHost — the on-reconnect drain
walks every row for the host, regardless of due-ness, since the
host being back makes 'due' irrelevant.

internal/server/http/schedule_push.go

@@ -206,8 +206,30 @@ func (s *Server) dispatchBackupForGroup(ctx context.Context, conn *ws.Conn, host
 	sendCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
 	defer cancel()
 	if err := conn.Send(sendCtx, env); err != nil {
-		slog.Warn("schedule.fire: send command.run",
-			"host_id", hostID, "schedule_id", scheduleID, "err", err)
+		slog.Warn("schedule.fire: send command.run failed, queueing for retry",
+			"host_id", hostID, "schedule_id", scheduleID, "group", g.Name, "err", err)
+		backoff := time.Duration(g.RetryBackoffSeconds) * time.Second
+		if backoff <= 0 {
+			backoff = 60 * time.Second
+		}
+		if enqueueErr := s.deps.Store.EnqueuePendingRun(ctx, &store.PendingRun{
+			ID:            ulid.Make().String(),
+			ScheduleID:    scheduleID,
+			SourceGroupID: g.ID,
+			HostID:        hostID,
+			Attempt:       1,
+			NextAttemptAt: time.Now().UTC().Add(backoff),
+			ScheduledAt:   scheduledAt,
+			LastError:     err.Error(),
+		}); enqueueErr != nil {
+			slog.Warn("schedule.fire: enqueue pending run failed",
+				"host_id", hostID, "schedule_id", scheduleID, "group", g.Name, "err", enqueueErr)
+		}
+		// The job row was already persisted earlier in this function — leave
+		// it in `queued` status. The drainer will re-dispatch (creating a
+		// new job row) and the orphaned queued row stays for forensic
+		// visibility. Don't delete it: the audit trail still wants to know
+		// "we tried and the wire was wedged."
 		return ""
 	}
 	_ = s.deps.Store.AppendAudit(ctx, store.AuditEntry{