fix: accept BytesIO in R2Storage.save() type hint

beartype was rejecting BytesIO objects passed to R2Storage.save()
because the BinaryIO type hint doesn't satisfy beartype's strict
protocol checking for BytesIO instances.

Changed type hint from `BinaryIO` to `BinaryIO | BytesIO` to
explicitly accept both types. This fixes playlist cover uploads
which use BytesIO to wrap image data.

Adds regression test that verifies BytesIO is accepted.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

backend/src/backend/storage/r2.py

···
 
 import time
 from collections.abc import Callable
+from io import BytesIO
 from pathlib import Path
 from typing import BinaryIO
 
···
 
     async def save(
         self,
-        file: BinaryIO,
+        file: BinaryIO | BytesIO,
         filename: str,
         progress_callback: Callable[[float], None] | None = None,
     ) -> str:
···
 
     async def save_gated(
         self,
-        file: BinaryIO,
+        file: BinaryIO | BytesIO,
         filename: str,
         progress_callback: Callable[[float], None] | None = None,
     ) -> str:

backend/tests/test_storage_types.py

···
+"""test storage type hints accept BytesIO.
+
+regression test for: https://github.com/zzstoatzz/plyr.fm/pull/736
+beartype was rejecting BytesIO for BinaryIO type hint in R2Storage.save()
+"""
+
+from io import BytesIO
+from unittest.mock import AsyncMock, patch
+
+from backend.storage.r2 import R2Storage
+
+
+async def test_r2_save_accepts_bytesio():
+    """R2Storage.save() should accept BytesIO objects.
+
+    BytesIO is the standard way to create in-memory binary streams,
+    and is used throughout the codebase for image uploads.
+
+    This test verifies that the type hint on save() is compatible
+    with BytesIO, which beartype validates at runtime.
+    """
+    # create a minimal image-like BytesIO
+    image_data = b"\x89PNG\r\n\x1a\n" + b"\x00" * 100  # fake PNG header
+    file_obj = BytesIO(image_data)
+
+    # mock the R2 client internals
+    with (
+        patch.object(R2Storage, "__init__", lambda self: None),
+        patch("backend.storage.r2.hash_file_chunked", return_value="abc123def456"),
+    ):
+        storage = R2Storage()
+        storage.async_session = AsyncMock()
+        storage.image_bucket_name = "test-images"
+        storage.audio_bucket_name = "test-audio"
+
+        # mock the async context manager for S3 client
+        mock_client = AsyncMock()
+        mock_client.upload_fileobj = AsyncMock()
+
+        mock_cm = AsyncMock()
+        mock_cm.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_cm.__aexit__ = AsyncMock(return_value=None)
+        storage.async_session.client = lambda *args, **kwargs: mock_cm
+        storage.endpoint_url = "https://test.r2.dev"
+        storage.aws_access_key_id = "test"
+        storage.aws_secret_access_key = "test"
+
+        # this should NOT raise a beartype error
+        # before the fix: BeartypeCallHintParamViolation
+        file_id = await storage.save(file_obj, "test.png")
+
+        assert file_id == "abc123def456"[:16]
+        mock_client.upload_fileobj.assert_called_once()