fix: atomically persist subtitle job state

[z2z23n0]

Problem

job.json was written directly with Path.write_text, which truncates the existing file before the replacement JSON is complete. An interrupted write can therefore leave invalid JSON, and the next startup silently skips that job.

Fix

• Write the complete payload to a unique temporary file in the job directory.
• Atomically replace job.json with os.replace only after the temporary write succeeds.
• Clean up temporary files on success and failure.
• Add regression coverage for interrupted writes, same-directory replacement, and restart loading with an orphan temporary file.

Tests

• python -m pytest -q tests/test_job_persistence.py (3 passed)
• python -m pytest -q (30 passed)