CONV-304: Add conversion cost estimate endpoint

[menvil]

Summary

• Adds POST /api/v1/conversions/estimate returning cost amount, breakdown, and has_enough_credits
• Uses EstimateConversionCostAction and CreditLedger — no job created, no credits spent
• Ownership enforced; unsupported pairs return 422
• Also adds ConversionController, ConversionResource, EstimateConversionRequest, CreateConversionRequest (used in CONV-305)

Test plan

• Returns amount/breakdown/has_enough_credits
• No conversion job created
• has_enough_credits false when balance < cost
• Other user's file → 403
• Unsupported format → 422 unsupported_conversion
• composer test passes (571 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds a cost estimate endpoint so users can see conversion pricing before creating a job. Fulfills Linear CONV-304 by returning amount, breakdown, and whether the user has enough credits, without spending credits.

• New Features
  • POST /api/v1/conversions/estimate returns amount, breakdown, and has_enough_credits.
  • Validates options via ConverterRegistry, uses EstimateConversionCostAction and CreditLedger; no job is created.
  • Enforces ownership (403). Unsupported pairs return 422 unsupported_conversion.
  • Adds ConversionController, ConversionResource, EstimateConversionRequest, and CreateConversionRequest (for CONV-305).

^{Written for commit 4d7334d. 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-304-add-conversion-cost-estimate-endpoint

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