feat: consolidate common_types, cross-contract tests, auth audit, coverage reporting

[demilade18-git]

Closes #1536, Closes #1537, Closes #1538, Closes #1539

• SC-20: Consolidated shared enums (BookingStatus, PaymentStatus, etc.) into common_types
• SC-21: Cross-contract integration tests (booking→payment→attendance flow)
• SC-22: Authorization audit documenting require_auth coverage across all entrypoints
• SC-23: cargo-deny + cargo-audit CI job for security scanning

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[yusuftomilola]

Excellent quality focus! Common types consolidation reduces duplication, cross-contract tests ensure integration reliability, auth audit strengthens security, and coverage reporting improves code quality visibility.