fix(runtime): add server-side sweeper to detect stale runtimes

The only path to marking a runtime offline was the daemon's deregister
call on graceful shutdown. If the daemon crashed, was killed, or lost
network, the status stayed "online" forever. Add a background goroutine
that sweeps every 30s and marks runtimes offline after 45s without a
heartbeat (3 missed intervals).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/cmd/server/main.go

@@ -65,6 +65,10 @@ func main() {
 		Handler: r,
 	}
 
+	// Start background sweeper to mark stale runtimes as offline.
+	sweepCtx, sweepCancel := context.WithCancel(context.Background())
+	go runRuntimeSweeper(sweepCtx, queries, bus)
+
 	// Graceful shutdown
 	go func() {
 		slog.Info("server starting", "port", port)
@@ -79,6 +83,7 @@ func main() {
 	<-quit
 
 	slog.Info("shutting down server")
+	sweepCancel()
 	shutdownCtx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 

server/cmd/server/runtime_sweeper.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"github.com/multica-ai/multica/server/internal/events"
+	"github.com/multica-ai/multica/server/internal/util"
+	db "github.com/multica-ai/multica/server/pkg/db/generated"
+	"github.com/multica-ai/multica/server/pkg/protocol"
+)
+
+const (
+	// sweepInterval is how often we check for stale runtimes.
+	sweepInterval = 30 * time.Second
+	// staleThresholdSeconds marks runtimes offline if no heartbeat for this long.
+	// The daemon heartbeat interval is 15s, so 45s = 3 missed heartbeats.
+	staleThresholdSeconds = 45.0
+)
+
+// runRuntimeSweeper periodically marks runtimes as offline if their
+// last_seen_at exceeds the stale threshold. This handles cases where the
+// daemon crashes or is killed without calling the deregister endpoint.
+func runRuntimeSweeper(ctx context.Context, queries *db.Queries, bus *events.Bus) {
+	ticker := time.NewTicker(sweepInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			staleRows, err := queries.MarkStaleRuntimesOffline(ctx, staleThresholdSeconds)
+			if err != nil {
+				slog.Warn("runtime sweeper: failed to mark stale runtimes offline", "error", err)
+				continue
+			}
+			if len(staleRows) == 0 {
+				continue
+			}
+
+			// Collect unique workspace IDs to notify.
+			workspaces := make(map[string]bool)
+			for _, row := range staleRows {
+				wsID := util.UUIDToString(row.WorkspaceID)
+				workspaces[wsID] = true
+			}
+
+			slog.Info("runtime sweeper: marked stale runtimes offline", "count", len(staleRows), "workspaces", len(workspaces))
+
+			// Notify frontend clients so they re-fetch runtime list.
+			for wsID := range workspaces {
+				bus.Publish(events.Event{
+					Type:        protocol.EventDaemonRegister,
+					WorkspaceID: wsID,
+					ActorType:   "system",
+					Payload: map[string]any{
+						"action": "stale_sweep",
+					},
+				})
+			}
+		}
+	}
+}