fix(backend): regroup offline sync segments before ASR

[waffensam]

Summary

• Regroup offline sync VAD WAVs into wider ASR windows before prerecorded transcription.
• Preserve wall-clock gaps with inserted silence, splitting windows when the gap is over 120s or the window would exceed 10 minutes.
• Apply the same path to v1 and v2 sync while reusing existing user transcription preferences and falling back to original VAD segments if regrouping fails.

Closes #8006

Testing

• /private/tmp/omi-black-venv/bin/black --line-length 120 --skip-string-normalization backend/routers/sync.py
• python3 -m py_compile backend/routers/sync.py
• git diff --check
• Not run: python3 -m pytest backend/tests/unit/test_sync_ordered_assignment.py -q (pytest is not installed in this environment)