feat: implement email template preview endpoint

[johdanike]

🎯 Summary

Implemented the POST endpoint to render email templates with injected sample data to support the preview functionality in routes-b.

📋 Technical Scope & Changes

• Created handler at app/api/routes-b/email-templates/[id]/preview/route.ts.
• Integrated authentication checks and enforced template ownership via userId.
• Added zod validation to parse and enforce structured sampleData payloads.
• Implemented Handlebars template compilation to return the rendered HTML string.
• Added comprehensive test coverage in tests/api/routes-b/email-templates/preview.test.ts for:
  • Happy path (200 OK + rendered HTML assertion).
  • Unauthenticated access (401).
  • Missing records (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 GitHub Actions run)

Closes #1129

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