T009: [P] Create CertificateAuditLog immutable entity

[Hemavathi15sg]

Phase 2: Foundational

Task: T009
Status: Not Started
Story: Foundational (blocking all user stories)
Parallelizable: Yes - independent from T007-T008, T010

Description

Create CertificateAuditLog immutable event log entity for all certificate lifecycle events.

Acceptance Criteria

• File created: api/CourseRegistration.Domain/Entities/CertificateAuditLog.cs
• All 8+ properties implemented (see data-model.md):
  • AuditLogId (Guid, PK)
  • CertificateId (Guid, FK)
  • EventType (enum: Issued, Revoked, Verified, Expired)
  • AdminUserId (int, FK, nullable)
  • EventTimestamp (datetime2, UTC)
  • Reason (string, nullable, for revocation reason)
  • EventDetails (string JSON, for extensibility)
  • IsDeleted (bit, soft delete)
  • CreatedUtc (datetime2)
• Immutability enforced: Property setters private or init-only
• EventType enum defined with all 4 event types
• Insert-only pattern: no modification/deletion allowed in domain logic
• XML documentation on public members

Effort Estimate

2.5 hours

References

• data-model.md: CertificateAuditLog Entity section
• research.md: Audit logging patterns