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>

backend/models/backup_schedule.py

@@ -19,6 +19,7 @@ class BackupSchedule(SQLModel, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
     stack_id: str
     destination_id: int
+    agent_id: Optional[int] = None  # None = local host; otherwise a remote agent
     frequency: str = "daily"  # one of FREQUENCIES
     hour: int = 3  # UTC, used for daily/weekly
     minute: int = 0
@@ -39,6 +40,7 @@ class BackupSchedule(SQLModel, table=True):
 class ScheduleCreate(SQLModel):
     stack_id: str
     destination_id: int
+    agent_id: Optional[int] = None
     frequency: str = "daily"
     hour: int = 3
     minute: int = 0
@@ -66,6 +68,8 @@ class ScheduleRead(SQLModel):
     stack_id: str
     destination_id: int
     destination_name: Optional[str]
+    agent_id: Optional[int]
+    agent_name: Optional[str]
     frequency: str
     hour: int
     minute: int