Phase 8: back up & restore remote (agent) stacks (0.8.0)

- Agent: GET /agent/stacks/{id}/backup + POST /agent/stacks/restore (reuse
  backup_service). backup_service gains backup_basename/backup_filename helpers.
- Main proxy streams agent <-> main <-> destination (creds stay central):
  agent_service download_to_file/upload_file; routers/agents.py backup download,
  backup/push, restore upload, restore-from.
- Schedules: BackupSchedule.agent_id; schedule_service downloads from the agent
  when set; per-host filename prefix isolates retention across hosts.
- Frontend: agents api backup/restore; BackupButton/RestoreButton agent-aware
  (Backup on remote stack detail, Restore per host section); schedule form host
  selector (local or an online agent) + host shown on schedule rows.

Rough-verified (per request): py_compile, frontend tsc build, image imports
(main 99 / agent 16 routes). Full live agent round-trip to be tested post-deploy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/RemoteStackDetail.tsx

@@ -14,6 +14,7 @@ import {
 import { toast } from "sonner";
 import { Badge, Button, Card, Spinner, StatusDot } from "@/components/ui";
 import { HostDot } from "@/components/hosts/HostDot";
+import { BackupButton } from "@/components/stacks/BackupRestore";
 import { agentsApi } from "@/api/agents";
 import { apiErrorMessage } from "@/api/client";
 import { useAuthStore } from "@/store/auth";
@@ -92,6 +93,7 @@ export function RemoteStackDetail() {
             <Button variant="outline" onClick={() => run("down", "Tearing down")} loading={busy}>
               <Power className="h-4 w-4" /> Down
             </Button>
+            <BackupButton stackId={id} agentId={aid} />
           </div>
         )}
       </div>