fix(update): close SQLite connection and protect cache dir during ZIP update

On Windows, shutil.rmtree() fails when deleting a directory that contains
an open SQLite database file. The ZIP update path in _download_and_replace_zip()
calls _clean_plugin_folder() which tries to delete the cache/ directory,
but downloaded_versions.sqlite is held open by DownloadedVersionHistoryService.

Fix:
- Add close() method to DownloadedVersionHistoryService to release
  the persistent SQLite connection
- Call close() before _clean_plugin_folder() in the ZIP update flow
- Add 'cache' to the skip_files list so the runtime cache directory is
  never deleted during plugin updates

py/services/downloaded_version_history_service.py

@@ -96,6 +96,21 @@ class DownloadedVersionHistoryService:
     def get_database_path(self) -> str:
         return self._db_path
 
+    def close(self) -> None:
+        """Close the persistent SQLite connection, if open.
+
+        This is called before plugin update operations to release the
+        database file lock on Windows, allowing ``shutil.rmtree()`` to
+        succeed when the cache resides inside the plugin directory.
+        """
+        if self._conn is not None:
+            try:
+                self._conn.close()
+            except Exception:
+                pass
+            finally:
+                self._conn = None
+
     def _get_active_library_name(self) -> str | None:
         try:
             value = self._settings.get_active_library_name()