fix: preserve saved subtitle style when rendering

[z2z23n0]

Problem

Subtitle style updates are persisted on the job, but rendering without a request body rebuilt SubtitleStyle from None. That silently restored default font size, margin, and speaker names instead of using the saved style.

Fix

• Reuse job.subtitle_style when the render request omits a style payload.
• Preserve the existing behavior where an explicit render payload overrides the saved style.
• Add an API regression test for both paths.

Tests

• python -m pytest -q tests/test_app_api.py (4 passed)
• python -m pytest -q (28 passed)