Use a temp file instead of io.ReadAll to avoid buffering the entire
Unsplash image in RAM, which could cause OOM with large images or
high maxsize configuration.
- Replace custom testfile structs with bytes.NewReader
- Remove readerOnly wrapper and non-seekable reader tests (no longer
possible at the type level)
- Update S3 unit tests to remove temp file assertions
Adds startup validation that checks if the configured file storage is writable. To do this, Vikunja now tries to create a temporary file and clean it up afterwards.
