CONV-310: Add API ownership guards

[menvil]

Summary

• Adds ApiOwnershipGuard with ensureFileOwner() and ensureConversionOwner() throwing AuthorizationException
• Refactors FileController and ConversionController to use guard instead of abort_if
• Extends ApiExceptionMapper to map 401/403/404 HttpExceptions to stable error codes (covers framework-converted exceptions)
• Security tests verify cross-user access is blocked on all relevant endpoints

Test plan

• File targets → 403 forbidden for non-owner
• Estimate → 403 forbidden for non-owner file
• Create conversion → 403 forbidden for non-owner file
• Conversion status → 403 forbidden for non-owner
• Conversion download → 403 forbidden for non-owner
• composer test passes (593 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds a centralized API ownership guard to block cross-user access and standardizes auth errors to stable codes. Addresses CONV-310 by enforcing file/conversion ownership and simplifying controllers.

• New Features

  • Added ApiOwnershipGuard with ensureFileOwner() and ensureConversionOwner() (throws AuthorizationException).
  • Extended ApiExceptionMapper to map 401/403/404 to codes: unauthorized/forbidden/not_found.
  • Security tests cover targets, estimate, create, status, and download for non-owner access.

• Refactors

  • Replaced inline abort_if checks in FileController and ConversionController with the guard methods.

^{Written for commit 06c4fb2. Summary will update on new commits.}

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