CONV-307: Add conversion result download endpoint

[menvil]

Summary

• Adds GET /api/v1/conversions/{conversion}/download streaming file download
• Checks: owner, status=completed, result file exists, not expired
• Non-completed → 422 result_not_ready; other user → 403
• Internal storage paths never exposed in JSON response

Test plan

• Completed conversion with result file → 200 download
• Queued conversion → 422 result_not_ready
• Other user → 403
• composer test passes (581 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds GET /api/v1/conversions/{conversion}/download to let users download completed conversion results. Streams the file and enforces ownership and completion per Linear CONV-307.

• New Features
  • Adds GET /api/v1/conversions/{conversion}/download that streams the result file.
  • Requires ownership, completed status, and an existing, non-expired result.
  • Returns 422 result_not_ready if not complete; 403 if not owner.
  • Hides internal storage paths; adds tests for success, 422, and 403.

^{Written for commit b6a6395. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-307-add-conversion-result-download-endpoint

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