feat: download completed docs via api (#1078)

## Description

Allow users to download a completed document via API.

## Testing Performed

Tested the code locally by trying to download both draft and completed
docs. Works as expected.

## Checklist

- [x] I have tested these changes locally and they work as expected.
- [ ] I have added/updated tests that prove the effectiveness of these
changes.
- [ ] I have updated the documentation to reflect these changes, if
applicable.
- [x] I have followed the project's coding style guidelines.
- [ ] I have addressed the code review feedback from the previous
submission, if applicable.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Implemented functionality to download signed documents directly from
the app.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/api/v1/contract.ts

@@ -11,6 +11,7 @@ import {
   ZDeleteDocumentMutationSchema,
   ZDeleteFieldMutationSchema,
   ZDeleteRecipientMutationSchema,
+  ZDownloadDocumentSuccessfulSchema,
   ZGetDocumentsQuerySchema,
   ZSendDocumentForSigningMutationSchema,
   ZSuccessfulDocumentResponseSchema,
@@ -51,6 +52,17 @@ export const ApiContractV1 = c.router(
       summary: 'Get a single document',
     },
 
+    downloadSignedDocument: {
+      method: 'GET',
+      path: '/api/v1/documents/:id/download',
+      responses: {
+        200: ZDownloadDocumentSuccessfulSchema,
+        401: ZUnsuccessfulResponseSchema,
+        404: ZUnsuccessfulResponseSchema,
+      },
+      summary: 'Download a signed document when the storage transport is S3',
+    },
+
     createDocument: {
       method: 'POST',
       path: '/api/v1/documents',