refactor(startup): move startup probe to gunicorn worker init

Move probe execution from Django app ready() to gunicorn.conf.py
Remove threading implementation to simplify startup sequence
Ensure probe runs in worker process context with proper error handling

mnemosyne/library/apps.py

@@ -199,17 +199,4 @@ class LibraryConfig(AppConfig):
     verbose_name = "Library"
 
     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:
-            # Never let the probe itself take down the process.
-            logger.warning("Startup probe crashed: %s", exc, exc_info=True)
+        pass