feat: wire backstory generation into NPC spawn flow

Trigger fire-and-forget backstory generation whenever an NPC spawns,
both during initial spawn and via player spawn-npc requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/network/SocketServer.ts

@@ -70,7 +70,8 @@ export class SocketServer {
 
         const target = map.findNearestWalkable(data.x, data.y, 3)
           ?? map.getRandomWalkable();
-        spawnNPC(world, map, target);
+        const npcEntity = spawnNPC(world, map, target);
+        this.gameLoop.generateNpcBackstory(npcEntity);
       });
 
       // Handle superlatives subscription