security: implement scholar identity verification anti-sybil measures

Scholars could create multiple wallets to claim multiple scholarships.\n\n**Action:**\n- [ ] Research Stellar-based identity solutions (e.g., Proof of Humanity, World ID)\n- [ ] Add optional identity verification step for scholarship applicants\n- [ ] Track verification status per address\n- [ ] Admin can flag suspicious addresses\n- [ ] Write ADR documenting the chosen approach and trade-offs