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>

cmd/server/main.go

@@ -103,4 +103,6 @@ func main() {
 	if err := httpServer.Shutdown(ctx); err != nil {
 		slog.Warn("HTTP server shutdown finished with error", "error", err)
 	}
+	// Let a running compaction finish before the deferred store.Close
+	srv.Close()
 }