feat: async sandbox lifecycle with Redis Stream events

Replace synchronous RPC-based CP-host communication for sandbox
lifecycle operations (Create, Pause, Resume, Destroy) with an async
pattern. CP handlers now return 202 Accepted immediately, fire agent
RPCs in background goroutines, and publish state events to a Redis
Stream. A background consumer processes events as a fallback writer.

Agent-side auto-pause events are pushed to the CP via HTTP callback
(POST /v1/hosts/sandbox-events), keeping Redis internal to the CP.

All DB status transitions use conditional updates
(UpdateSandboxStatusIf, UpdateSandboxRunningIf) to prevent race
conditions between concurrent operations and background goroutines.

The HostMonitor reconciler is kept at 60s as a safety net, extended
to handle transient statuses (starting, pausing, resuming, stopping).

Frontend updated to handle 202 responses with empty bodies and render
transient statuses with blue indicators.

internal/api/host_monitor.go

@@ -213,4 +213,49 @@ func (m *HostMonitor) checkHost(ctx context.Context, host db.Host) {
 			slog.Warn("host monitor: failed to mark stopped", "host_id", id.FormatHostID(host.ID), "error", err)
 		}
 	}
+
+	// --- Reconcile transient statuses (starting, resuming, pausing, stopping) ---
+	// These represent in-flight operations. If the sandbox is no longer alive on
+	// the host, infer the final state based on the transient status.
+
+	transientSandboxes, err := m.db.ListSandboxesByHostAndStatus(ctx, db.ListSandboxesByHostAndStatusParams{
+		HostID:  host.ID,
+		Column2: []string{"starting", "resuming", "pausing", "stopping"},
+	})
+	if err != nil {
+		slog.Warn("host monitor: failed to list transient sandboxes", "host_id", id.FormatHostID(host.ID), "error", err)
+		return
+	}
+
+	for _, sb := range transientSandboxes {
+		sbIDStr := id.FormatSandboxID(sb.ID)
+		if _, ok := alive[sbIDStr]; ok {
+			// Sandbox is alive on host — the background goroutine should
+			// finalize the transition. For starting/resuming, if the sandbox
+			// is alive it means creation/resume succeeded.
+			if sb.Status == "starting" || sb.Status == "resuming" {
+				if _, err := m.db.UpdateSandboxStatusIf(ctx, db.UpdateSandboxStatusIfParams{
+					ID: sb.ID, Status: sb.Status, Status_2: "running",
+				}); err == nil {
+					slog.Info("host monitor: promoted transient sandbox to running", "sandbox_id", sbIDStr, "from", sb.Status)
+				}
+			}
+			continue
+		}
+		// Sandbox is not alive on host — infer final state.
+		var finalStatus string
+		switch sb.Status {
+		case "starting", "resuming":
+			finalStatus = "error"
+		case "pausing":
+			finalStatus = "paused"
+		case "stopping":
+			finalStatus = "stopped"
+		}
+		if _, err := m.db.UpdateSandboxStatusIf(ctx, db.UpdateSandboxStatusIfParams{
+			ID: sb.ID, Status: sb.Status, Status_2: finalStatus,
+		}); err == nil {
+			slog.Info("host monitor: resolved transient sandbox", "sandbox_id", sbIDStr, "from", sb.Status, "to", finalStatus)
+		}
+	}
 }