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/folder_trash.py

@@ -0,0 +1,70 @@
+"""Empty a browser-test media folder without waiting for the disk (#384).
+
+A reset used to ``shutil.rmtree`` the output and input folders the last
+fixture wrote. Unlinking a file whose pages are still being written back
+waits for that write, so on a slow CI disk a reset stalled in ``rmtree``.
+``replace_folder`` renames the old folder into ``<parent>/reset-trash`` (one
+metadata change) and makes a new empty one; a background thread deletes the
+trash, and ``close_trash`` stops that thread and removes what is left before
+the server's temp root goes.
+"""
+
+from __future__ import annotations
+
+import queue
+import shutil
+import threading
+import uuid
+from pathlib import Path
+
+TRASH_NAME = "reset-trash"
+
+
+class _Trash:
+    def __init__(self, directory: Path) -> None:
+        self.directory = directory
+        self._queue: "queue.SimpleQueue[Path | None]" = queue.SimpleQueue()
+        self._thread = threading.Thread(
+            target=self._work, name="pm-reset-trash", daemon=True
+        )
+        self._thread.start()
+
+    def _work(self) -> None:
+        while (folder := self._queue.get()) is not None:
+            shutil.rmtree(folder, ignore_errors=True)
+
+    def discard(self, folder: Path) -> None:
+        self.directory.mkdir(exist_ok=True)
+        moved = self.directory / f"{folder.name}-{uuid.uuid4().hex}"
+        folder.rename(moved)
+        self._queue.put(moved)
+
+    def close(self) -> None:
+        self._queue.put(None)
+        self._thread.join()
+        shutil.rmtree(self.directory, ignore_errors=True)
+
+
+_trashes: dict[Path, _Trash] = {}
+_lock = threading.Lock()
+
+
+def replace_folder(folder: Path) -> None:
+    """Make ``folder`` an empty folder; its old content is deleted later."""
+    folder = Path(folder)
+    if folder.exists():
+        directory = folder.parent / TRASH_NAME
+        with _lock:
+            trash = _trashes.get(directory)
+            if trash is None:
+                trash = _trashes[directory] = _Trash(directory)
+            trash.discard(folder)
+    folder.mkdir()
+
+
+def close_trash(parent: Path) -> None:
+    """Stop deleting under ``parent``'s trash and remove it (blocking)."""
+    with _lock:
+        trash = _trashes.pop(Path(parent) / TRASH_NAME, None)
+    if trash is not None:
+        trash.close()