refactor: move startup probe to daemon thread with 10s timeout

Move the _run_startup_probe logic into a separate daemon thread
within LibraryConfig.ready. This prevents indefinite blocking on
startup while maintaining a 10-second wait for the probe result.

mnemosyne/library/apps.py

@@ -201,6 +201,13 @@ class LibraryConfig(AppConfig):
     def ready(self):
         if _should_skip_probe():
             return
+        import threading
+
+        t = threading.Thread(target=self._probe_thread, daemon=True)
+        t.start()
+        t.join(timeout=10)
+
+    def _probe_thread(self):
         try:
             _run_startup_probe()
         except Exception as exc: