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/docsync.go

@@ -266,9 +266,14 @@ func (s *Server) handleDocLeave(w http.ResponseWriter, r *http.Request) {
 const compactionInterval = 6 * time.Hour
 
 func (s *Server) startCompaction() {
+	defer s.bg.Done()
 	for {
 		s.compactOnce()
-		time.Sleep(compactionInterval)
+		select {
+		case <-s.stop:
+			return
+		case <-time.After(compactionInterval):
+		}
 	}
 }