test(01-01): add TestUpdateEvent_PreservesTagOnUpsert regression test

- Verifies tag survives a second UpdateEvent() for the same image (DATA-01)
- Verifies acknowledged_at is reset to NULL by the new event
- Verifies event fields (Status) are updated by the new event

pkg/diunwebhook/diunwebhook_test.go

@@ -612,3 +612,56 @@ func TestGetUpdates_IncludesTag(t *testing.T) {
 		t.Errorf("expected tag id %d, got %d", id, entry.Tag.ID)
 	}
 }
+
+func TestUpdateEvent_PreservesTagOnUpsert(t *testing.T) {
+	diun.UpdatesReset()
+
+	// Insert image
+	if err := diun.UpdateEvent(diun.DiunEvent{Image: "nginx:latest", Status: "new"}); err != nil {
+		t.Fatalf("first UpdateEvent failed: %v", err)
+	}
+
+	// Assign tag
+	tagID := postTagAndGetID(t, "webservers")
+	body, _ := json.Marshal(map[string]interface{}{"image": "nginx:latest", "tag_id": tagID})
+	req := httptest.NewRequest(http.MethodPut, "/api/tag-assignments", bytes.NewReader(body))
+	rec := httptest.NewRecorder()
+	diun.TagAssignmentHandler(rec, req)
+	if rec.Code != http.StatusNoContent {
+		t.Fatalf("tag assignment failed: got %d", rec.Code)
+	}
+
+	// Dismiss (acknowledge) the image — second event must reset this
+	req = httptest.NewRequest(http.MethodPatch, "/api/updates/nginx:latest", nil)
+	rec = httptest.NewRecorder()
+	diun.DismissHandler(rec, req)
+	if rec.Code != http.StatusNoContent {
+		t.Fatalf("dismiss failed: got %d", rec.Code)
+	}
+
+	// Receive a second event for the same image
+	if err := diun.UpdateEvent(diun.DiunEvent{Image: "nginx:latest", Status: "update"}); err != nil {
+		t.Fatalf("second UpdateEvent failed: %v", err)
+	}
+
+	// Tag must survive the second event
+	m := diun.GetUpdatesMap()
+	entry, ok := m["nginx:latest"]
+	if !ok {
+		t.Fatal("nginx:latest missing from updates after second event")
+	}
+	if entry.Tag == nil {
+		t.Error("tag was lost after second UpdateEvent — UPSERT bug not fixed")
+	}
+	if entry.Tag != nil && entry.Tag.ID != tagID {
+		t.Errorf("tag ID changed: expected %d, got %d", tagID, entry.Tag.ID)
+	}
+	// Acknowledged state must be reset by the new event
+	if entry.Acknowledged {
+		t.Error("acknowledged state must be reset by new event")
+	}
+	// Status must reflect the new event
+	if entry.Event.Status != "update" {
+		t.Errorf("expected status 'update', got %q", entry.Event.Status)
+	}
+}