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/schema.ts

@@ -53,6 +53,10 @@ export const ZUploadDocumentSuccessfulSchema = z.object({
   key: z.string(),
 });
 
+export const ZDownloadDocumentSuccessfulSchema = z.object({
+  downloadUrl: z.string(),
+});
+
 export type TUploadDocumentSuccessfulSchema = z.infer<typeof ZUploadDocumentSuccessfulSchema>;
 
 export const ZCreateDocumentMutationSchema = z.object({