Worker logs expected PreviewError failures as warnings; tracebacks are
reserved for unanticipated internal errors. pdf read errors report
backend "pdf" (vips not yet reached); the stage field is dropped as the
backend name now encodes the failing pipeline step. Backend label
"pyvips" renamed to "vips" everywhere.
Add fixtures and tests covering image EXIF orientations, HDR AVIF, SDR video, 90/270 rotated video, HDR video with and without rotation, PDF, dispatch, and OnlyOffice (skipped unless configured).
Regenerate rotated/HDR video fixtures and sample.pdf with tests/files/generate_fixtures.py.
LeoVasanko