feat: add patient consent screen and backend enforcement

[princessoladele]

• Add patient_consent table to SQLite schema (wallet, consented_at)
• Add GET /v1/patient/consent/:wallet and POST /v1/patient/consent endpoints
• Enforce consent before minting: vaccination/issue returns 403 if patient has not consented (waivable via REQUIRE_PATIENT_CONSENT=false env var)
• Add ConsentScreen modal component explaining on-chain data visibility
• Add useConsent hook for checking and recording consent
• Show ConsentScreen to first-time patients in PatientDashboard
• Add REQUIRE_PATIENT_CONSENT to .env.example

Closes #118

Description

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

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