Phase 7: scheduled (recurring) backups (0.7.0)

- BackupSchedule model + schedule_service: next-run calc (hourly/daily/weekly,
  UTC), background scheduler loop (lifespan), run-one with retention pruning
  (keep newest N per stack on the destination), backup_failed notify event.
- routers/schedules.py: schedules CRUD + run-now; registered in main.py.
- Frontend: api/schedules.ts + Settings → Scheduled backups (list with next/last
  run + status, enable/disable, run-now, delete; add form with stack/destination/
  frequency/time/weekday/retention/volumes).

Rough-verified only (per request): py_compile, frontend tsc build, app import
(95 routes), next-run math sanity. Full live run to be tested after deploy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/models/setting.py

@@ -16,6 +16,7 @@ EVENT_STACK_START = "stack_start"
 EVENT_STACK_STOP = "stack_stop"
 EVENT_STACK_ERROR = "stack_error"
 EVENT_PULL_FAILED = "pull_failed"
+EVENT_BACKUP_FAILED = "backup_failed"
 
 ALL_EVENTS = [
     EVENT_UPDATE_AVAILABLE,
@@ -23,6 +24,7 @@ ALL_EVENTS = [
     EVENT_STACK_STOP,
     EVENT_STACK_ERROR,
     EVENT_PULL_FAILED,
+    EVENT_BACKUP_FAILED,
 ]
 
 WEBHOOK_TYPES = ["ntfy", "discord", "slack", "gotify", "generic"]