feat(ansible): structured playbook-variable fields on the schedule form

The Ansible Schedules form only offered a free-form extra-vars textarea,
unlike the browse/host run forms which auto-populate a field per declared
playbook variable (vars:/vars_prompt:) plus the playbook description. Wire
the schedule form to the same shared `_playbook_vars.html` partial:

- Playbook <select> now loads /ansible/playbook-vars?schedule=1 on change
  into a #sch-playbook-vars container (mirrors the host detail form).
- Edit mode pre-renders the saved playbook's variables server-side with
  their stored values; the extra-vars textarea keeps only leftover
  (non-declared) vars.
- Partial gains optional `values` (prefill) and `schedule` flags. In the
  schedule context secret vars render disabled ("secrets can't be
  scheduled") since they're dropped downstream, and the destructive-confirm
  gate is shown but not required.

Backend already parsed var__* fields for schedules, so this is the
front-end/partial parity fix.

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

steward/templates/ansible/schedules.html

@@ -75,6 +75,8 @@
 
 {# ── Create / edit form ───────────────────────────────────────────────────── #}
 {% set p = (editing.params or {}) if editing else {} %}
+{% set saved_vars = p.get('extra_vars_map') or {} %}
+{% set declared_names = (edit_variables or []) | map(attribute='name') | list %}
 <div class="card" id="form">
   <h2 class="section-title" style="margin-bottom:1rem;">{{ "Edit schedule" if editing else "New schedule" }}</h2>
   <form method="post" action="{{ '/ansible/schedules/' ~ editing.id if editing else '/ansible/schedules' }}">
@@ -96,7 +98,9 @@
       </div>
       <div class="form-group" style="margin-bottom:0;">
         <label>Playbook</label>
-        <select name="playbook_path" id="sch-playbook" required>
+        <select name="playbook_path" id="sch-playbook" required
+                hx-get="/ansible/playbook-vars?schedule=1" hx-trigger="change"
+                hx-target="#sch-playbook-vars" hx-swap="innerHTML" hx-include="closest form">
           <option value="">— choose playbook —</option>
           {% for pb in all_playbooks %}
           <option value="{{ pb }}" {% if editing and editing.playbook_path == pb %}selected{% endif %}>{{ pb }}</option>
@@ -132,11 +136,29 @@
         <input type="text" name="tags" value="{{ p.get('tags', '') }}" placeholder="tag1,tag2">
       </div>
     </div>
-    <div class="form-group" style="margin-top:1rem;">
-      <label>Extra vars (one key=value per line)</label>
-      <textarea name="extra_vars" rows="3" placeholder="prune_volumes=false">{% for k, v in (p.get('extra_vars_map') or {}).items() %}{{ k }}={{ v }}
-{% endfor %}</textarea>
+
+    {# Declared playbook variables (vars:/vars_prompt:) — loaded via HTMX on
+       playbook change; pre-rendered here in edit mode so saved values show. #}
+    <div id="sch-playbook-vars" style="margin-top:1rem;">
+      {% if editing %}
+      {% set variables = edit_variables %}
+      {% set description = edit_description %}
+      {% set category = edit_category %}
+      {% set confirm = edit_confirm %}
+      {% set values = saved_vars %}
+      {% set schedule = true %}
+      {% include "ansible/_playbook_vars.html" %}
+      {% endif %}
     </div>
+
+    <details style="margin-top:0.75rem;">
+      <summary style="cursor:pointer;font-size:0.82rem;color:var(--text-muted);">Extra vars</summary>
+      <div class="form-group" style="margin-top:0.6rem;">
+        <label>Extra vars <span style="color:var(--text-muted);font-weight:normal;">(one key=value per line — for vars not listed above)</span></label>
+        <textarea name="extra_vars" rows="3" placeholder="prune_volumes=false">{% for k, v in saved_vars.items() if k not in declared_names %}{{ k }}={{ v }}
+{% endfor %}</textarea>
+      </div>
+    </details>
     <div class="form-group" style="display:flex;align-items:center;gap:0.5rem;">
       <input type="checkbox" name="check" id="check" style="width:auto;" {% if p.get('check') %}checked{% endif %}>
       <label for="check" style="margin-bottom:0;">Dry run (--check --diff)</label>