Feature: Get letter PDF


Implement the following endpoint:

| Method | Endpoint | Success Code | Error Codes | Description |
| -- | -- | -- | -- | -- |
GET | /letter/{id} | 200 OK | 404 | Return the letter as PDF using the ID of the corresponding template.

- Protocol: JSON over HTTP.
- Headers: All requests sending data and all responses must include Content-Type: application/json.