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/services/backup_service.py

@@ -38,6 +38,26 @@ class BackupError(Exception):
     pass
 
 
+# --------------------------------------------------------------------------- #
+# Backup filename convention
+# --------------------------------------------------------------------------- #
+
+
+def backup_basename(stack_id: str, prefix: Optional[str] = None) -> str:
+    """Filename stem used to group a stack's backups (and match for retention).
+
+    ``prefix`` (e.g. a remote host slug) keeps backups of same-named stacks on
+    different hosts from colliding / pruning each other on a shared destination.
+    """
+    return f"backup-{prefix}-{stack_id}" if prefix else f"backup-{stack_id}"
+
+
+def backup_filename(stack_id: str, include_volumes: bool, prefix: Optional[str] = None) -> str:
+    date = now().strftime("%Y%m%d-%H%M%S")
+    suffix = "full" if include_volumes else "config"
+    return f"{backup_basename(stack_id, prefix)}-{suffix}-{date}.tar.gz"
+
+
 # --------------------------------------------------------------------------- #
 # Helper container for volume I/O
 # --------------------------------------------------------------------------- #