Combat tracker: multiple named encounters

- Encounter entity; Combatant now belongs to an encounter (ON DELETE
  CASCADE). Migration adds encounter_id, rehoming any existing combatants.
- /tracker lists encounters (name, count, last updated) with a "New
  encounter" form; /tracker/{id} is the HP tracker for one encounter.
- Encounters can be renamed, have their combatants cleared, or be deleted.
- The Enemies list gains an encounter picker next to "Add to tracker"
  (existing encounter or "— New encounter —", which prompts for a name);
  an encounter's "Add enemies" link pre-selects it via ?encounter=.
- Routes for combatant HP/remove moved under /tracker/combatant/ to keep
  the numeric {id} for encounters.

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

templates/enemy/index.html.twig

@@ -5,10 +5,16 @@
 {% block body %}
     <div class="d-flex justify-content-between align-items-center mb-3 gap-2 flex-wrap">
         <h1 class="h3 mb-0">Enemies</h1>
-        <div class="d-flex gap-2"
+        <div class="d-flex gap-2 align-items-center flex-wrap"
              data-controller="tracker-picker"
              data-tracker-picker-url-value="{{ path('app_tracker_add') }}"
              data-tracker-picker-token-value="{{ csrf_token('track-add') }}">
+            <select class="form-select form-select-sm encounter-select" style="width: auto;" aria-label="Encounter">
+                <option value="new"{{ selected_encounter is null ? ' selected' }}>— New encounter —</option>
+                {% for e in encounters %}
+                    <option value="{{ e.id }}"{{ selected_encounter == e.id ? ' selected' }}>{{ e.name }}</option>
+                {% endfor %}
+            </select>
             <button type="button" class="btn btn-outline-primary btn-sm" data-action="tracker-picker#submit">
                 Add to tracker
             </button>