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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "mindsift-frontend",
   "private": true,
-  "version": "0.37.0",
+  "version": "0.37.1",
   "license": "Unlicense",
   "type": "module",
   "scripts": {