feat: implement embeddable SVG badges, credential revocation, and audit logging (Phase 4)

[ashishsinghbora]

What changed

• Built dynamic SVG badge generation endpoint (/api/badges/credential/[id]/badge.svg) allowing contributors to embed verifiable proof of work on their GitHub profile README.
• Added copyable GitHub Markdown badge snippet component to the public credential verification page (/verify/[id]).
• Implemented secure administrative credential revocation API at /api/admin/credentials/revoke with mandatory reasoning and audit logging to the audit_logs table.
• Added administrative revocation modal and action on the verification page for authorized admins.
• Added visible revocation banner and rationale display for revoked credentials.
• Added comprehensive unit and integration tests for SVG badge generation, width calculations, and credential revocation audit logging.

Verification

• Vitest: 11/11 test suites, 32/32 tests passing.
• TypeScript tsc --noEmit: 0 errors.
• ESLint: 0 errors, 0 warnings.
• Production build: all 14 routes compiled and optimized.