feat(media): add public URL generation for media files and improve S3 integration

Introduce `getPublicUrl` in `S3Service` for generating public URLs. Replace custom file URL generation logic across services with the new method. Add media controller for file streaming and update related tests. Adjust frontend to display user roles instead of email in the sidebar. Update environment schema to include optional `API_URL`. Fix help page contact email.

frontend/src/components/app-sidebar.tsx

@@ -228,7 +228,7 @@ export function AppSidebar() {
 											<span className="truncate font-semibold">
 												{user.displayName || user.username}
 											</span>
-											<span className="truncate text-xs">{user.email}</span>
+											<span className="truncate text-xs">{user.role}</span>
 										</div>
 										<ChevronRight className="ml-auto size-4 group-data-[collapsible=icon]:hidden" />
 									</SidebarMenuButton>