fix(server): stop the background compaction before the database is closed

The compaction starts right away in NewServer and could still write to the SQLite files while a
test's temp dir was removed ("directory not empty" on the CI runner, TestWebhookTokenInURLOnlyAPIKey).
Server.Close stops it and waits for a running pass; tests call it before closing the store, and the
server calls it on shutdown before the deferred store.Close. v0.37.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/demo_test.go

@@ -26,6 +26,7 @@ func TestDemoResetKeepsSessionsAndRestoresSampleData(t *testing.T) {
 		t.Fatal(err)
 	}
 	srv := NewServer(&config.Config{DatabasePath: dbPath, JWTSecret: "demo-secret", DemoMode: true}, store, ws.NewHub(), nil)
+	t.Cleanup(srv.Close)
 	router := srv.Router()
 	call := func(method, path, token, body string) *httptest.ResponseRecorder {
 		req := httptest.NewRequest(method, path, strings.NewReader(body))