CONV-305: Add create conversion endpoint

[menvil]

Summary

• Adds POST /api/v1/conversions delegating to CreateConversionJobAction
• Returns 201 Created with status/source_format/target_format
• 402 insufficient_credits when balance < cost
• 403 when file not owned by API user

Test plan

• Creates conversion job with status queued
• Insufficient credits → 402 insufficient_credits
• Other user's file → 403
• Missing API key → 401
• composer test passes (575 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds POST /api/v1/conversions to create a conversion job and return the job details. Implements auth, credit checks, and file ownership rules to meet Linear CONV-305.

• New Features
  • POST /api/v1/conversions creates a job via CreateConversionJobAction and returns 201 with status, source_format, and target_format.
  • Validates auth, credits, and file ownership with 401, 402 (insufficient_credits), and 403 responses.
  • Feature tests cover queued job creation and all error cases.

^{Written for commit 6a48de3. 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-305-add-create-conversion-endpoint

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