Remove the ASYNC230 per-file ignore in voice.py

[alimaandev]

Problem

pyproject.toml ignores ASYNC230 (blocking calls inside async functions) for shortube/voice.py. The async _generate_with_timestamps path performs blocking file I/O, which blocks the event loop and is a latent bug for any future async caller.

Task

Rewrite the async path so no blocking calls run inside the coroutine (or drop async entirely and use a plain function run in a worker). Remove the per-file ignore and the comment in pyproject.toml.

Acceptance criteria

• python -m ruff check shortube tests is clean with the per-file ignore removed
• tests/test_voice.py passes unchanged (keep the public generate_voiceover API)
• Voiceover output is byte-identical for the same inputs (regression-safe)

Files

• shortube/voice.py
• pyproject.toml
• tests/test_voice.py