feat(ansible): run a playbook against a single Steward host (task 547)

From a host's edit page, an operator can run a playbook against just that host
via an ephemeral one-host inventory — no need for the host to exist in a source
inventory.

- ansible/sources.py: pure host_inventory_content(host) -> '<name> ansible_host=<addr>'
- executor.start_run: optional inventory_content written to the temp dir and used
  as -i (overrides inventory_path); cleaned up with the creds dir
- hosts/routes.py: POST /hosts/<id>/run-playbook (operator) — validates source +
  playbook, builds the ephemeral inventory, starts a user-triggered run, redirects
  to the live run detail; edit page gets the ansible source list
- hosts/form.html: 'Run Ansible playbook against this host' panel (shown when
  editing an existing host and sources exist) — source + playbook + extra-vars/
  tags/dry-run (no --limit; single host)
- tests: unit host_inventory_content; integration runs a hosts:all/connection:local
  playbook against the ephemeral inventory and asserts inventory_hostname

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

steward/templates/hosts/form.html

@@ -49,5 +49,43 @@
             </div>
         </form>
     </div>
+
+    {% if host and ansible_sources %}
+    <div class="card" style="margin-top:1.5rem;">
+        <h3 class="section-title" style="margin-bottom:0.5rem;">Run Ansible playbook against this host</h3>
+        <p style="color:var(--text-muted);font-size:0.82rem;margin-bottom:1rem;">
+            Runs against an ephemeral one-host inventory for
+            <code>{{ host.name }}</code> ({{ host.address or "no address — resolves by name" }}).
+            Use a playbook with <code>hosts: all</code>.
+        </p>
+        <form method="post" action="/hosts/{{ host.id }}/run-playbook">
+            <div class="form-group">
+                <label>Source</label>
+                <select name="source_name" required>
+                    {% for s in ansible_sources %}<option value="{{ s }}">{{ s }}</option>{% endfor %}
+                </select>
+            </div>
+            <div class="form-group">
+                <label>Playbook <span style="color:var(--text-muted);font-weight:normal;">(path within source)</span></label>
+                <input type="text" name="playbook_path" placeholder="playbooks/site.yml" required>
+            </div>
+            <div class="form-group">
+                <label>Extra vars <span style="color:var(--text-muted);font-weight:normal;">(optional, one key=value per line)</span></label>
+                <textarea name="extra_vars" rows="2"
+                          style="width:100%;font-family:ui-monospace,monospace;font-size:0.85rem;"></textarea>
+            </div>
+            <div class="form-group">
+                <label>Tags <span style="color:var(--text-muted);font-weight:normal;">(optional)</span></label>
+                <input type="text" name="tags">
+            </div>
+            <div class="form-group">
+                <label style="display:flex;align-items:center;gap:0.5rem;font-weight:normal;">
+                    <input type="checkbox" name="check"> Dry-run (--check --diff)
+                </label>
+            </div>
+            <button type="submit" class="btn">Run playbook</button>
+        </form>
+    </div>
+    {% endif %}
 </div>
 {% endblock %}