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>

src/Repository/EncounterRepository.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Repository;
+
+use App\Entity\Encounter;
+use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
+use Doctrine\Persistence\ManagerRegistry;
+
+/**
+ * @extends ServiceEntityRepository<Encounter>
+ */
+class EncounterRepository extends ServiceEntityRepository
+{
+    public function __construct(ManagerRegistry $registry)
+    {
+        parent::__construct($registry, Encounter::class);
+    }
+
+    /** @return list<Encounter> most-recent first */
+    public function recent(): array
+    {
+        return $this->findBy([], ['updatedAt' => 'DESC', 'id' => 'DESC']);
+    }
+}