Show AC on each tracker row

Combatant snapshots armor_class from the enemy when added (like maxHp),
shown as an "AC n" pill next to the name for quick hit checks. Migration
backfills existing combatants from their enemy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/tracker/show.html.twig

@@ -51,6 +51,7 @@
                             <div class="col-12 col-md-4">
                                 <div class="fw-semibold {{ c.isDown ? 'text-decoration-line-through' : '' }}">
                                     {{ c.label }}
+                                    <span class="badge rounded-pill text-bg-secondary ms-1" title="Armor Class">AC {{ c.armorClass }}</span>
                                     {% if c.isDown %}<span class="badge text-bg-secondary ms-1">Down</span>{% endif %}
                                 </div>
                                 {% if c.enemy %}