[WIP] Update draft issue placeholder with additional details

[Copilot]

Plan: Update Certificate Entity to Comply with Certificate Creation Rules

• Analyze current Certificate entity and identify missing fields per .github/instructions/certifcate.instructions.md
• Update Certificate entity to include all required fields:
  • Add SerialNumber (unique, sequential, human-readable)
  • Add HolderName (derived from Student name, validated, sanitized)
  • Add CourseTitle (from Course name)
  • Rename IssueDate to IssueDateUtc (ensure UTC)
  • Add ExpiryDateUtc (nullable)
  • Add IssuedBy (authority identifier)
  • Add VerificationUrl (HTTPS endpoint)
  • Rename/update DigitalSignature to SignatureHash (SHA-256)
  • Add Version (semantic version)
  • Add Status (Active | Revoked | Expired)
• Add revocation fields (RevocationReason, RevokedDateUtc)
• Update validation attributes to match rules
• Update CertificateDto to match new entity structure
• Update CertificateService to implement generation workflow
• Add certificate verification endpoint controller
• Run tests to ensure no breaking changes
• Run CodeQL security checks
• Request code review

Original prompt

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This section details on the original issue you should resolve

<issue_title>Draft issue placeholder</issue_title>
<issue_description>This is a draft issue created to capture a new request. Please provide additional details so the issue can be completed.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Draft issue placeholder #23

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