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/Entity/Combatant.php

@@ -19,6 +19,10 @@ class Combatant
     #[ORM\Column]
     private ?int $id = null;
 
+    #[ORM\ManyToOne(targetEntity: Encounter::class, inversedBy: 'combatants')]
+    #[ORM\JoinColumn(name: 'encounter_id', nullable: false, onDelete: 'CASCADE')]
+    private ?Encounter $encounter = null;
+
     #[ORM\ManyToOne(targetEntity: Enemy::class)]
     #[ORM\JoinColumn(name: 'enemy_id', onDelete: 'SET NULL', nullable: true)]
     private ?Enemy $enemy = null;
@@ -73,6 +77,18 @@ class Combatant
         return $this->id;
     }
 
+    public function getEncounter(): ?Encounter
+    {
+        return $this->encounter;
+    }
+
+    public function setEncounter(?Encounter $encounter): static
+    {
+        $this->encounter = $encounter;
+
+        return $this;
+    }
+
     public function getEnemy(): ?Enemy
     {
         return $this->enemy;