Create learning credential data model

[MyouzzZ]

Summary

• define learner identity, achievement, evidence/proof, badge metadata, and future Soroban record fields
• map current Study Copilot progress/session signals into an MVP credential model
• document privacy requirements for wallet linking, evidence hashes, and public credentials

Closes #31.

Validation

• Reviewed README Stellar/Soroban roadmap and Drips application text
• Reviewed current progress/session/bookmark signals in backend/main.py and study-copilot-full.html
• git diff --check -- docs/learning-credential-data-model.md