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/templates/ansible/index.html

@@ -3,7 +3,10 @@
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1.5rem;">
     <h1 class="page-title" style="margin-bottom:0;">Ansible Runs</h1>
-    <a href="/ansible/browse" class="btn">Browse Playbooks</a>
+    <div style="display:flex;gap:0.5rem;">
+      <a href="/ansible/schedules" class="btn btn-ghost">Schedules</a>
+      <a href="/ansible/browse" class="btn">Browse Playbooks</a>
+    </div>
 </div>
 {% if not runs %}
 <div class="card">