feat: add scaling mode stretch

[jasongdove]

Resolves #70

[gemini-code-assist]

Code Review

This pull request introduces a ScalingMode configuration, allowing users to choose between ScaleAndPad and Stretch during video normalization. The changes include updates to the configuration schema, the FFmpeg pipeline logic, and hardware acceleration modules to support these modes. A review comment identifies a logic error where scaling is skipped if dimensions match, noting that anamorphic content still requires scaling to normalize the sample aspect ratio.