feat: add downloadable release management for desktop client

- Extend DesktopClientReleaseCheckResponse with platform, arch, and channel fields.
- Introduce DesktopClientDownloadableRelease and DesktopClientDownloadableReleaseListResponse interfaces for managing downloadable releases.
- Implement ListDownloadable method in DesktopClientReleaseService to retrieve downloadable releases based on the specified channel.
- Update DesktopClientReleaseCheckResult to include platform, arch, and channel information.
- Add resolveInstallerDownloadURL method to handle download URL resolution for releases.
- Create new endpoint in DesktopReleaseHandler for listing downloadable releases.
- Update router to include the new downloadable releases route.
- Enhance tests to cover the new functionality for resolving download URLs and listing downloadable releases.

server/internal/tenant/transport/router.go

@@ -98,6 +98,9 @@ func RegisterRoutes(app *bootstrap.App) {
 	media := tenantProtected.Group("/media")
 	media.GET("/platforms", mediaHandler.ListPlatforms)
 
+	desktopClient := tenantProtected.Group("/desktop-client")
+	desktopClient.GET("/releases/downloadable", desktopReleaseHandler.ListDownloadable)
+
 	mediaSupplyHandler := NewMediaSupplyHandler(app)
 	mediaSupply := tenantProtected.Group("/media-supply")
 	mediaSupply.GET("/resources", mediaSupplyHandler.ListResources)