Add family tree page, linked from the NPC's family name

- /npcs/family/{family} (app_npc_family) renders a parent->child tree for
  everyone sharing that family name; co-parents who are both in the family
  are paired into one couple node, elders first
- FamilyTreeBuilder does the flat-list -> tree assembly
- the NPC detail page's family name (subtitle + Details row) now links there
- .family-tree CSS: nested list with connector lines, small portrait avatars

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

templates/npc/show.html.twig

@@ -12,7 +12,9 @@
             <div>
                 <a href="{{ path('app_npcs') }}" class="text-decoration-none small">&larr; All NPC's</a>
                 <h1 class="h3 mb-0">{{ npc.name }}</h1>
-                <span class="text-body-secondary">{{ npc.family }}{% if npc.occupation %} · {{ npc.occupation }}{% endif %}</span>
+                <span class="text-body-secondary">
+                    <a href="{{ path('app_npc_family', {family: npc.family}) }}">{{ npc.family }}</a>{% if npc.occupation %} · {{ npc.occupation }}{% endif %}
+                </span>
             </div>
         </div>
         <a href="{{ path('app_npc_edit', {id: npc.id}) }}" class="btn btn-outline-secondary btn-sm">Edit details</a>
@@ -24,7 +26,8 @@
                 <div class="card-header">Details</div>
                 <div class="card-body">
                     <dl class="row mb-0">
-                        <dt class="col-5">Family</dt><dd class="col-7">{{ npc.family }}</dd>
+                        <dt class="col-5">Family</dt>
+                        <dd class="col-7"><a href="{{ path('app_npc_family', {family: npc.family}) }}">{{ npc.family }}</a> family tree</dd>
                         <dt class="col-5">Age</dt><dd class="col-7">{{ npc.age ?? '—' }}</dd>
                         <dt class="col-5">Race</dt><dd class="col-7">{{ npc.race.value }}</dd>
                         <dt class="col-5">Occupation</dt><dd class="col-7">{{ npc.occupation ?? '—' }}</dd>