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/routers/schedules.py

@@ -6,6 +6,7 @@ from sqlmodel import Session, select
 
 from auth import require_admin
 from database import get_session
+from models.agent import Agent
 from models.backup_destination import BackupDestination
 from models.backup_schedule import (
     FREQUENCIES,
@@ -27,11 +28,14 @@ def _ip(request: Request) -> str:
 
 def _to_read(session: Session, s: BackupSchedule) -> ScheduleRead:
     dest = session.get(BackupDestination, s.destination_id)
+    agent = session.get(Agent, s.agent_id) if s.agent_id is not None else None
     return ScheduleRead(
         id=s.id,
         stack_id=s.stack_id,
         destination_id=s.destination_id,
         destination_name=dest.name if dest else None,
+        agent_id=s.agent_id,
+        agent_name=agent.name if agent else None,
         frequency=s.frequency,
         hour=s.hour,
         minute=s.minute,
@@ -54,13 +58,17 @@ def _get_or_404(session: Session, schedule_id: int) -> BackupSchedule:
     return s
 
 
-def _validate(session: Session, stack_id: str, destination_id: int, frequency: str) -> None:
-    if frequency not in FREQUENCIES:
-        raise HTTPException(status_code=400, detail=f"Unknown frequency '{frequency}'")
-    if not session.get(Stack, stack_id):
-        raise HTTPException(status_code=404, detail=f"Stack '{stack_id}' not found")
-    if not session.get(BackupDestination, destination_id):
-        raise HTTPException(status_code=404, detail=f"Destination {destination_id} not found")
+def _validate(session: Session, schedule: BackupSchedule) -> None:
+    if schedule.frequency not in FREQUENCIES:
+        raise HTTPException(status_code=400, detail=f"Unknown frequency '{schedule.frequency}'")
+    if not session.get(BackupDestination, schedule.destination_id):
+        raise HTTPException(status_code=404, detail=f"Destination {schedule.destination_id} not found")
+    if schedule.agent_id is not None:
+        # Remote stack: validate the agent exists; the stack is checked at run time.
+        if not session.get(Agent, schedule.agent_id):
+            raise HTTPException(status_code=404, detail=f"Agent {schedule.agent_id} not found")
+    elif not session.get(Stack, schedule.stack_id):
+        raise HTTPException(status_code=404, detail=f"Stack '{schedule.stack_id}' not found")
 
 
 @router.get("", response_model=list[ScheduleRead])
@@ -79,8 +87,8 @@ def create_schedule(
     session: Session = Depends(get_session),
     user: User = Depends(require_admin),
 ) -> ScheduleRead:
-    _validate(session, body.stack_id, body.destination_id, body.frequency)
     s = BackupSchedule(**body.model_dump())
+    _validate(session, s)
     s.next_run = schedule_service.compute_next_run(
         s.frequency, s.hour, s.minute, s.weekday, schedule_service._now()
     )
@@ -106,7 +114,7 @@ def update_schedule(
     data = body.model_dump(exclude_unset=True)
     for k, v in data.items():
         setattr(s, k, v)
-    _validate(session, s.stack_id, s.destination_id, s.frequency)
+    _validate(session, s)
     # Recompute next run when timing fields change.
     if {"frequency", "hour", "minute", "weekday"} & set(data) or s.next_run is None:
         s.next_run = schedule_service.compute_next_run(