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/__init__.py

@@ -2,6 +2,7 @@
 from models.agent import Agent
 from models.audit import AuditLog
 from models.backup_destination import BackupDestination
+from models.backup_schedule import BackupSchedule
 from models.setting import Setting, Webhook
 from models.stack import Stack
 from models.template import Template
@@ -9,5 +10,5 @@ from models.user import User
 
 __all__ = [
     "User", "Stack", "AuditLog", "Template", "Setting", "Webhook", "Agent",
-    "BackupDestination",
+    "BackupDestination", "BackupSchedule",
 ]