Scaffolding failure leaves an orphaned project directory

[groupthinking]

Problem

ProjectCodeGenerator.generate_project creates a temporary project directory and
hands the path back to its caller. If scaffolding raises, the handler logs and
re-raises without removing the directory, so the path is never returned and the
directory has no owner.

Aspect	Detail
File	src/youtube_extension/backend/code_generator.py
Directory created	tempfile.mkdtemp(prefix="uvai_project_")
Handler	except Exception as e: logger.error(...); raise (L276-278)
Missing	No removal of the directory the function created
Age	Pre-existing; predates #1250 / #1251

The caller at backend/services/video_processing_service.py:380 reads
generation_result["project_path"] at L390, so it never learns the path when the
call raises. There is no rmtree anywhere in that module.

Reachability evidence

POST /api/v1/video-to-software        router.py:778   (mounted main.py:192)
  -> process_video_to_software        video_processing_service.py:317
  -> code_generator.generate_project  video_processing_service.py:380

Same endpoint verified in #1239 and #1250.

Scope note

This is deliberately not fixed in #1251. That PR restores cancellation
atomicity after moving scaffolding writes onto worker threads, and it covers the
CancelledError path only. The generic-exception leak is older, is not caused by
that change, and touching the shared handler risks altering existing failure
expectations. Splitting it keeps the perf change reviewable.

Acceptance criteria

• A scaffolding failure removes the directory generate_project created.
• The original exception still propagates unchanged; cleanup never masks it.
• Cleanup runs off the event loop and drains any worker still writing, matching
  the _run_to_completion / _discard_project_dir helpers added in perf: batch project scaffolding disk writes off the event loop #1251.
• A regression test forces a real failure (no mocks on the mechanism under test)
  and asserts no uvai_project_* directory survives.

Proposed fix

Reuse the helpers introduced in #1251: on the exception path, await _discard_project_dir(project_path) before re-raising, guarded so a cleanup
failure cannot replace the original exception.

[linear-code]

GRV-253

[github-actions]

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