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/app.py

@@ -284,6 +284,21 @@ def _register_core_tasks(app: Quart) -> None:
             )
         )
 
+    # Fire due Ansible schedules (recurring playbook runs). Checks every minute;
+    # each schedule's own interval gates whether it actually runs.
+    async def run_ansible_schedules():
+        from .ansible.scheduler import run_due_schedules
+        await run_due_schedules(app)
+
+    app._task_registry.append(
+        ScheduledTask(
+            name="ansible_scheduled_runs",
+            coro_factory=run_ansible_schedules,
+            interval_seconds=60,
+            run_on_startup=False,
+        )
+    )
+
 
 async def _mark_interrupted_runs(app: Quart) -> None:
     from sqlalchemy import update