FIX: Removing output_path paramater from video converter

[Richard Lundeen (rlundeen2)]

Summary

• Remove the caller-controlled output_path parameter from AddImageVideoConverter.
• Change video_path to Path, consistent with other converter input files.
• Write generated video bytes through data_serializer_factory and return its managed path.
• Add registry support for coercing and exposing Path parameters to the backend UI.

Rationale

Allowing callers to select output_path lets UI-controlled input choose an arbitrary server-side write destination. A path traversal value could overwrite files outside the results store, and even valid-looking paths can cause unexpected filesystem side effects.

Selecting storage destinations is also outside converter scope. The converter should transform input into output. The serializer owns output location, storage backend selection, and persistence. Routing the result through data_serializer_factory removes the arbitrary-write surface and aligns this converter with the other file-producing converters.

This parameter is removed directly rather than deprecated because retaining it would preserve the security issue.

Validation

• uv run --no-sync pytest tests/unit/converter/test_add_image_video_converter.py tests/unit/backend/test_converter_service.py tests/unit/models/test_parameter.py -q
• uv run --no-sync pre-commit run --all-files