feat(ansible): scheduled recurring playbook runs

Adds cron-like recurring runs (the engine the maintenance-automation work
needs). New AnsibleSchedule model + migration 0018; a core ScheduledTask
(ansible_scheduled_runs, 60s) fires due schedules, each creating a
system-triggered AnsibleRun (triggered_by=None). Centralises the
resolve-inventory → create-run → launch flow in ansible/runner.trigger_run,
shared by the manual route (refactored to use it) and the scheduler.

Schedules UI under /ansible/schedules: create/edit/pause/delete/run-now,
with interval presets, scope targeting (all / group / target), extra-vars /
limit / tags / dry-run, and last-run status (resolved via last_run_id) +
next-run. Unit test for the due-check.

Task #549 (milestone #37).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/models/__init__.py

@@ -6,6 +6,7 @@ from .metrics import PluginMetric
 from .alerts import AlertRule, AlertState, AlertEvent, AlertOperator, AlertStateEnum
 from .ansible import AnsibleRun, AnsibleRunStatus
 from .ansible_inventory import AnsibleTarget, AnsibleGroup, ansible_target_groups
+from .ansible_schedule import AnsibleSchedule
 from .settings import AppSetting
 from .dashboard import Dashboard, DashboardWidget, DashboardShareToken
 
@@ -17,6 +18,7 @@ __all__ = [
     "AlertRule", "AlertState", "AlertEvent", "AlertOperator", "AlertStateEnum",
     "AnsibleRun", "AnsibleRunStatus",
     "AnsibleTarget", "AnsibleGroup", "ansible_target_groups",
+    "AnsibleSchedule",
     "AppSetting",
     "Dashboard", "DashboardWidget", "DashboardShareToken",
 ]