fix(hub): stop launching system updaters after manager shutdown (#1950) (#1951)

The staggered Initialize goroutine kept calling AddSystem after the hub
had been torn down, which spawned StartUpdater goroutines that later
dereferenced a nil DB inside PocketBase. Track a manager-level context
that RemoveAllSystems cancels, abort the staggered loop on shutdown,
and refuse AddSystem once the manager is stopped.

internal/hub/systems/system_test.go

@@ -3,9 +3,11 @@
 package systems
 
 import (
+	"context"
 	"testing"
 
 	"github.com/henrygd/beszel/internal/entities/system"
+	"github.com/pocketbase/pocketbase/tools/store"
 )
 
 func TestCombinedData_MigrateDeprecatedFields(t *testing.T) {
@@ -157,3 +159,20 @@ func TestCombinedData_MigrateDeprecatedFields(t *testing.T) {
 		}
 	})
 }
+
+// TestAddSystemRefusedAfterShutdown verifies that AddSystem returns an error
+// once the manager has been shut down, so the staggered Initialize starter
+// cannot spawn updater goroutines against a torn-down hub. See #1950.
+func TestAddSystemRefusedAfterShutdown(t *testing.T) {
+	sm := &SystemManager{systems: store.New(map[string]*System{})}
+	sm.ctx, sm.cancel = context.WithCancel(context.Background())
+	sm.cancel()
+
+	err := sm.AddSystem(&System{Id: "sys", Host: "127.0.0.1"})
+	if err == nil {
+		t.Fatalf("AddSystem returned nil error after shutdown")
+	}
+	if sm.systems.Has("sys") {
+		t.Fatalf("system was added to store after shutdown")
+	}
+}