run Check DB in a background thread

Since the DB is now stored behind a mutex, we're no longer limited
to accessing the database on the main thread.

qt/aqt/main.py

@@ -12,6 +12,7 @@ import signal
 import time
 import zipfile
 from argparse import Namespace
+from concurrent.futures import Future
 from threading import Thread
 from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple
 
@@ -1260,9 +1261,12 @@ will be lost. Continue?"""
 
     def onCheckDB(self):
         "True if no problems"
-        self.progress.start(immediate=True)
-        ret, ok = self.col.fixIntegrity()
+        self.progress.start()
+
+        def onDone(future: Future):
             self.progress.finish()
+            ret, ok = future.result()
+
             if not ok:
                 showText(ret)
             else:
@@ -1278,7 +1282,8 @@ will be lost. Continue?"""
                 except Exception as e:
                     print("swallowed exception in reset hook:", e)
                     continue
-        return ret
+
+        self.taskman.run_in_background(self.col.fixIntegrity, onDone)
 
     def on_check_media_db(self) -> None:
         check_media_db(self)