test(browser): empty media folders by a rename, delete them in the background (#384)

With the database commits gone, throttled resets still stalled in rmtree:
unlinking the last fixture's files waits for their pending writeback. Each
reseed now moves the old output and input folders into <root>/reset-trash
and makes empty ones; a background thread deletes the trash, and the
server's cleanup stops it and removes the rest before the temp root goes.

tests/browser/workspace_library_editor_fixture.py

@@ -3,11 +3,11 @@
 from __future__ import annotations
 
 from pathlib import Path
-import shutil
 from typing import TYPE_CHECKING
 
 from aiohttp import web
 from PIL import Image
+from tests.browser.folder_trash import replace_folder
 
 if TYPE_CHECKING:
     from py.api import PromptManagerAPI
@@ -51,8 +51,7 @@ def seed_library_editor(api: PromptManagerAPI, root: Path) -> dict:
         for table in _TABLES:
             conn.execute(f"DELETE FROM {table}")
     for name in ("output", "input"):
-        shutil.rmtree(root / name)
-        (root / name).mkdir()
+        replace_folder(root / name)
     epoch = db.get_execution_epoch()
     receipt = db.create_library_recipe(
         EDITOR_POSITIVE,