feat: implement email template versions endpoints (GET/POST)

[johdanike]

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🎯 Summary

Implemented the GET and POST endpoints to list and create versions for a specific email template under routes-b.

📋 Technical Scope & Changes

• Created route handler at app/api/routes-b/email-templates/[id]/versions/route.ts.
• GET: Retrieves all versions associated with a template ID, ordered by creation date descending.
• POST: Creates a new version payload. Includes zod validation enforcing subject and content.
• Integrated PrivyClient (verifyAuthToken) to authenticate the request via Bearer token and enforce template ownership using claims.userId.
• Added comprehensive test coverage in tests/api/routes-b/email-templates/versions.test.ts checking:
  • Happy paths (200 OK for GET, 201 Created for POST).
  • Validation failures (400).
  • Unauthenticated access (401).
  • Missing records (404).
  • Ownership violations (403).

✅ Acceptance Criteria

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

Closes #1130

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