fix(render): avoid shared concat list file

[NgoQuocViet2001]

Summary

• Write the ffmpeg concat demuxer list to a per-call temp file in edit_dir instead of the fixed _concat.txt path.
• Clean the temp concat list in a finally block so failed ffmpeg runs do not leave stale list files behind.
• Add a stdlib unittest covering unique concat-list names and cleanup.

Why

Issue #64 calls out that concurrent renders in the same edit directory can clobber the shared _concat.txt file. Keeping the temp file inside edit_dir preserves the concat demuxer's filesystem behavior while avoiding cross-render overwrites.

Test plan

• python -m unittest tests.test_render_concat
• python -m unittest discover -s tests
• python -m py_compile helpers/render.py tests/test_render_concat.py

---

Summary by cubic

Write the ffmpeg concat list to a unique temp file per render in edit_dir and delete it after the run. This avoids _concat.txt collisions during concurrent renders and prevents stale files.

• Bug Fixes
  • Use tempfile.NamedTemporaryFile to create _concat_*.txt in edit_dir.
  • Remove the temp list in a finally block, even if ffmpeg fails.
  • Add tests/test_render_concat.py to verify unique names and cleanup.

^{Written for commit 0f2a674. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}