fix(server): storage prefix url (#15477)

#### PR Dependency Tree


* **PR #15477** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

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

* **New Features**
* Added support for custom signed download URLs with configurable CDN
origins, object paths, expiration times, and security signatures.
* Custom download signing is available when both the URL prefix and
signing key are configured.

* **Bug Fixes**
* Preserved custom URL and signing settings when generating download
links.
  * Added validation for invalid signing configurations.
* Standard S3 storage continues using its existing presigned URL
behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/base/storage/providers/index.ts

@@ -127,12 +127,12 @@ const S3ConfigSchema: JSONSchema = {
         urlPrefix: {
           type: 'string',
           description:
-            'Optional custom origin for browser upload URLs. Provider presigned URLs also use this origin when signKey is not configured.',
+            'Optional base URL for proxied uploads and signed custom GET URLs. Provider presigned uploads require an origin-only value when signKey is not configured.',
         },
         signKey: {
           type: 'string',
           description:
-            'Optional HMAC key for signed upload URLs. Without urlPrefix, upload URLs use the server origin.',
+            'Optional HMAC key for signed upload and custom GET URLs. Custom GET URLs require both signKey and urlPrefix.',
         },
       },
     },