feat(backend,frontend): copy liked songs into a shareable playlist

backend/pyproject.toml

@@ -8,6 +8,8 @@ authors = [
 requires-python = ">=3.13"
 dependencies = [
     "fastapi>=0.139.0",
+    "httpx>=0.28.1",
+    "itsdangerous>=2.2.0",
     "pydantic-settings>=2.14.2",
     "rich>=15.0.0",
     "uvicorn[standard]>=0.50.0",