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/schedules.ts

@@ -5,6 +5,8 @@ export interface BackupSchedule {
   stack_id: string;
   destination_id: number;
   destination_name: string | null;
+  agent_id: number | null;
+  agent_name: string | null;
   frequency: "hourly" | "daily" | "weekly";
   hour: number;
   minute: number;
@@ -22,6 +24,7 @@ export interface BackupSchedule {
 export interface ScheduleInput {
   stack_id: string;
   destination_id: number;
+  agent_id?: number | null;
   frequency: string;
   hour: number;
   minute: number;