Fix upload intent validation

[ItsJazii]

Summary

• Project upload-session requests back to the strict artifact intent shape.
• Preserve strict metadata validation while allowing the required showcase ownership field.
• Add regression coverage for the upload request boundary.

Verification

• npm run phase2:preflight && npx tsc --noEmit && npm run lint && npm test
• Reproduced on staging before the fix: image upload returned Invalid upload metadata.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.