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/api/agents.ts

@@ -27,4 +27,69 @@ export const agentsApi = {
     api.put(`/api/agents/${id}/stacks/${stackId}`, body).then((r) => r.data),
   action: (id: number, stackId: string, action: string) =>
     api.post(`/api/agents/${id}/stacks/${stackId}/${action}`).then((r) => r.data),
+
+  backupDownload: async (
+    id: number,
+    stackId: string,
+    opts: { includeVolumes: boolean; stopFirst: boolean }
+  ) => {
+    const res = await api.get(`/api/agents/${id}/stacks/${stackId}/backup`, {
+      params: { include_volumes: opts.includeVolumes, stop_first: opts.stopFirst },
+      responseType: "blob",
+    });
+    const cd = res.headers["content-disposition"] as string | undefined;
+    const name = cd?.match(/filename="?([^"]+)"?/)?.[1] ?? `backup-${stackId}.tar.gz`;
+    const url = URL.createObjectURL(res.data as Blob);
+    const a = document.createElement("a");
+    a.href = url;
+    a.download = name;
+    document.body.appendChild(a);
+    a.click();
+    a.remove();
+    URL.revokeObjectURL(url);
+  },
+  backupPush: (
+    id: number,
+    stackId: string,
+    body: { destination_id: number; include_volumes: boolean; stop_first: boolean }
+  ) =>
+    api
+      .post<{ ok: boolean; destination: string; name: string }>(
+        `/api/agents/${id}/stacks/${stackId}/backup/push`,
+        body
+      )
+      .then((r) => r.data),
+  restoreUpload: (
+    id: number,
+    file: File,
+    opts: { targetId?: string; overwrite: boolean; restoreVolumes: boolean }
+  ) => {
+    const form = new FormData();
+    form.append("file", file);
+    if (opts.targetId) form.append("target_id", opts.targetId);
+    form.append("overwrite", String(opts.overwrite));
+    form.append("restore_volumes", String(opts.restoreVolumes));
+    return api
+      .post<{ stack_id: string; name: string; volumes_restored: number }>(
+        `/api/agents/${id}/stacks/restore`,
+        form
+      )
+      .then((r) => r.data);
+  },
+  restoreFrom: (
+    id: number,
+    body: {
+      destination_id: number;
+      name: string;
+      target_id?: string;
+      overwrite: boolean;
+      restore_volumes: boolean;
+    }
+  ) =>
+    api
+      .post<{ stack_id: string; name: string; volumes_restored: number }>(
+        `/api/agents/${id}/stacks/restore-from`,
+        body
+      )
+      .then((r) => r.data),
 };