Fix SubHD downloads and low-confidence subtitle output

[IronJKZ]

Summary

• update SubHD downloads for the current prepared-page protocol
• retry explicit low-quality ffsubsync alignments without the skip flag
• deliver the best usable Chinese subtitle even when confidence remains low
• reject unparseable originals and synchronized outputs
• report confidence, candidate, selection mode, and timeline score
• update English and Chinese documentation

Root cause

SubHD now requires POST /api/sub/prepare-download before opening /down/{sid}; direct access returns an expired-page 403. Separately, fixsub treated low confidence as an output veto, so usable candidates were retained but never applied.

Validation

• 230 tests passed after rebasing onto the latest origin/main
• sdist and wheel built successfully
• git diff --check is clean
• live SubHD probe found two results for Black Cat White Cat (1998) and downloaded the 2160p candidate ZIP
• independent task reviews and whole-branch review completed with no remaining findings