feat(backend): add queue system; add retry system

tests/test_reorder_and_delete.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from src.app.core.models import PlaylistItem, SyncAction, SyncActionType
+from src.app.core.sync.executor import ActionExecutor
+from src.app.core.sync.reorder import safe_multi_rename
+
+
+def test_safe_multi_rename_swaps_files(tmp_path: Path):
+    a = tmp_path / "0001 - A.mp4"
+    b = tmp_path / "0002 - B.mp4"
+    a.write_text("A", encoding="utf-8")
+    b.write_text("B", encoding="utf-8")
+
+    safe_multi_rename([(a, b), (b, a)])
+
+    assert (tmp_path / "0001 - A.mp4").read_text(encoding="utf-8") == "B"
+    assert (tmp_path / "0002 - B.mp4").read_text(encoding="utf-8") == "A"
+
+
+def test_executor_deletes_to_recycle(tmp_path: Path):
+    class StubDB:
+        def clear_file_state(self, playlist_id: str, video_id: str) -> None:
+            return None
+
+    executor = ActionExecutor(StubDB())  # type: ignore[arg-type]
+
+    save_root = tmp_path / "downloads"
+    audio_root = save_root / "audio"
+    video_root = save_root / "video"
+    audio_root.mkdir(parents=True, exist_ok=True)
+    video_root.mkdir(parents=True, exist_ok=True)
+
+    victim = audio_root / "0001 - X.mp3"
+    victim.write_text("x", encoding="utf-8")
+
+    item = PlaylistItem(playlist_id="p", video_id="v", title="t", playlist_index=1, local_filename=victim.name, downloaded=True)
+    action = SyncAction(SyncActionType.DELETE, item=item, from_name=victim.name)
+
+    executor._apply_deletions([action], audio_root, video_root, {"url": "p"})  # type: ignore[attr-defined]
+
+    assert not victim.exists()
+    recycled = save_root / ".recycle" / "audio" / victim.name
+    assert recycled.exists()
+