feat: implement email template version activation endpoint

[johdanike]

🎯 Summary

Implemented the POST endpoint to activate a specific version of an email template in routes-b.

📋 Technical Scope & Changes

• Created handler at app/api/routes-b/email-templates/[id]/versions/[v]/activate/route.ts.
• Integrated verifyAuthToken from Privy to authenticate the user and enforce template ownership.
• Added validation checks to ensure the requested version exists and is properly linked to the parent template.
• Implemented the Prisma update logic to set the newly activated version.
• Added comprehensive unit tests in tests/api/routes-b/email-templates/activate-version.test.ts covering:
  • Happy path (200 OK + database update assertion).
  • Unauthenticated/invalid token requests (401).
  • Parent template or target version not found (404).
  • Mismatched template/version relationship (404).
  • Ownership violations (403).

✅ Acceptance Criteria

• Handler implemented for POST method at the documented path.
• Validation and error envelopes match existing routes conventions.
• Unit tests pass in CI. (Pending)

Closes #1131

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