Fix #67: Add WriteJSON helper to httpserver package

[mishankov]

Summary

Closes #67

Adds a WriteJSON helper function to the httpserver package for writing JSON HTTP responses.

Changes

• Added WriteJSON(ctx context.Context, w http.ResponseWriter, statusCode int, data any) error function
• Function sets Content-Type: application/json header
• Writes the specified HTTP status code
• Encodes the data as JSON to the response writer
• Logs encoding errors via log.ErrorContext
• Returns wrapped error on encoding failures

Testing

• Added comprehensive tests in response_test.go covering:
  • JSON response with custom status code
  • Struct encoding
  • Error handling on unencodable data
• All tests pass: go test ./httpserver/...
• Linting clean: golangci-lint run reports 0 issues

Code Review

No P0/P1 blockers found. Implementation follows existing patterns in the auth handlers.