Fix flaky t.TempDir cleanup race in webhook tests (closes #198)

HandleWebhook starts a deployment in a detached goroutine that writes
under the app data directory, which is the tests t.TempDir; the tests
slept 100ms and returned, racing Go automatic TempDir cleanup and
intermittently failing with RemoveAll: directory not empty. The webhook
Service now tracks those goroutines in a sync.WaitGroup and exposes
WaitForDeployments; the tests wait on it instead of sleeping. Production
behavior is unchanged apart from making completion observable.

Model: opus-4-8

internal/service/webhook/webhook_test.go

@@ -7,7 +7,6 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
-	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -828,8 +827,9 @@ func TestExtractBranch(testingT *testing.T) {
 			)
 			require.NoError(t, err)
 
-			// Allow async deployment goroutine to complete before test cleanup
-			time.Sleep(100 * time.Millisecond)
+			// Wait for the async deployment goroutine to finish so its
+			// writes under the temp dir complete before test cleanup.
+			svc.WaitForDeployments()
 
 			events, err := app.GetWebhookEvents(context.Background(), 10)
 			require.NoError(t, err)
@@ -867,8 +867,9 @@ func TestHandleWebhookMatchingBranch(t *testing.T) {
 	)
 	require.NoError(t, err)
 
-	// Allow async deployment goroutine to complete before test cleanup
-	time.Sleep(100 * time.Millisecond)
+	// Wait for the async deployment goroutine to finish so its writes
+	// under the temp dir complete before test cleanup.
+	svc.WaitForDeployments()
 
 	events, err := app.GetWebhookEvents(context.Background(), 10)
 	require.NoError(t, err)
@@ -962,8 +963,9 @@ func assertHandleWebhookDeploys(
 	err := svc.HandleWebhook(context.Background(), app, source, pushEventType, payload)
 	require.NoError(t, err)
 
-	// Allow async deployment goroutine to complete before test cleanup
-	time.Sleep(100 * time.Millisecond)
+	// Wait for the async deployment goroutine to finish so its writes
+	// under the temp dir complete before test cleanup.
+	svc.WaitForDeployments()
 
 	events, err := app.GetWebhookEvents(context.Background(), 10)
 	require.NoError(t, err)