feat(scheduler): add auto_backup, auto_stop, auto_down, auto_start and delete_after_run one-shot mode (#777)

* feat(scheduler): add auto_backup, auto_stop, auto_down, auto_start actions and delete_after_run one-shot mode

Extends the scheduler with four new stack-targeted actions:

- auto_backup: backs up stack compose files and .env using the existing
  FileSystemService.backupStackFiles primitive
- auto_stop: runs compose stop (containers preserved)
- auto_down: runs compose down (containers removed)
- auto_start: runs compose up -d via deployStack (universal start for both
  stopped and down stacks)

Adds delete_after_run boolean column to scheduled_tasks. When enabled,
the task self-deletes after its first successful execution; failures keep
the task so the user can debug and retry.

All four new actions gate at Admiral tier, consistent with restart/snapshot/prune.
Migration is idempotent (maybeAddCol).

* docs(scheduler): update scheduled-operations doc with new lifecycle actions and delete-after-run

Adds the four new actions (Backup Stack Files, Stop Stack, Take Stack Down,
Start Stack) to the action table. Documents the delete-after-run one-shot mode
with its success-only deletion semantics. Adds the Stack Lifecycle Scheduling
section explaining stop-vs-down semantics and the local-execution boundary.
Adds three troubleshooting entries: auto-start on a missing compose folder,
auto-backup single-slot overwrite by design, and one-shot task disappearing
after successful run.

Updates the timeline description from four to five lanes. Refreshes
screenshots to show the new dialog layout with the Lifecycle lane visible.

frontend/src/types/scheduling.ts

@@ -4,7 +4,7 @@ export interface ScheduledTask {
   target_type: 'stack' | 'fleet' | 'system';
   target_id: string | null;
   node_id: number | null;
-  action: 'restart' | 'snapshot' | 'prune' | 'update' | 'scan';
+  action: 'restart' | 'snapshot' | 'prune' | 'update' | 'scan' | 'auto_backup' | 'auto_stop' | 'auto_down' | 'auto_start';
   cron_expression: string;
   enabled: number;
   created_by: string;
@@ -17,6 +17,7 @@ export interface ScheduledTask {
   prune_targets: string | null;
   target_services: string | null;
   prune_label_filter: string | null;
+  delete_after_run?: number;
   next_runs?: number[];
 }