feat: add backstory generation helper for NPC LLM backstories

Adds formatStatsForPrompt to convert Stats into prompt-friendly abbreviations
and generateBackstory to call the LLM service with the backstory template,
writing the result back to the entity's backstory component.

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

server/src/llm/__tests__/backstoryGenerator.test.ts

@@ -0,0 +1,89 @@
+import { describe, it, expect, vi } from 'vitest';
+import { World } from '../../ecs/World.js';
+import { formatStatsForPrompt, generateBackstory } from '../backstoryGenerator.js';
+import type { Stats } from '@dflike/shared';
+import type { LlmService } from '../llmService.js';
+
+describe('formatStatsForPrompt', () => {
+  it('formats stats into a readable string', () => {
+    const stats: Stats = {
+      strength: 15, dexterity: 12, constitution: 14,
+      intelligence: 8, perception: 10,
+      sociability: 6, courage: 16, curiosity: 11,
+      empathy: 7, temperament: 13,
+    };
+    const result = formatStatsForPrompt(stats);
+    expect(result).toBe(
+      'STR:15, DEX:12, CON:14, INT:8, PER:10, SOC:6, COU:16, CUR:11, EMP:7, TMP:13'
+    );
+  });
+});
+
+describe('generateBackstory', () => {
+  it('calls LLM service with correct template and variables', async () => {
+    const world = new World();
+    const entity = world.createEntity();
+    world.addComponent(entity, 'name', 'Brynn');
+    world.addComponent(entity, 'backstory', '');
+    world.addComponent<Stats>(entity, 'stats', {
+      strength: 15, dexterity: 12, constitution: 14,
+      intelligence: 8, perception: 10,
+      sociability: 6, courage: 16, curiosity: 11,
+      empathy: 7, temperament: 13,
+    });
+
+    const mockService: LlmService = {
+      generate: vi.fn().mockResolvedValue('A brave warrior from the north.'),
+      queueDepth: () => 0,
+      clear: () => {},
+    };
+
+    await generateBackstory(world, entity, mockService);
+
+    expect(mockService.generate).toHaveBeenCalledWith('backstory', {
+      npcName: 'Brynn',
+      stats: 'STR:15, DEX:12, CON:14, INT:8, PER:10, SOC:6, COU:16, CUR:11, EMP:7, TMP:13',
+    });
+
+    expect(world.getComponent<string>(entity, 'backstory')).toBe(
+      'A brave warrior from the north.'
+    );
+  });
+
+  it('leaves backstory empty when LLM returns null', async () => {
+    const world = new World();
+    const entity = world.createEntity();
+    world.addComponent(entity, 'name', 'Thom');
+    world.addComponent(entity, 'backstory', '');
+    world.addComponent<Stats>(entity, 'stats', {
+      strength: 10, dexterity: 10, constitution: 10,
+      intelligence: 10, perception: 10,
+      sociability: 10, courage: 10, curiosity: 10,
+      empathy: 10, temperament: 10,
+    });
+
+    const mockService: LlmService = {
+      generate: vi.fn().mockResolvedValue(null),
+      queueDepth: () => 0,
+      clear: () => {},
+    };
+
+    await generateBackstory(world, entity, mockService);
+
+    expect(world.getComponent<string>(entity, 'backstory')).toBe('');
+  });
+
+  it('does not crash if entity is missing components', async () => {
+    const world = new World();
+    const entity = world.createEntity();
+
+    const mockService: LlmService = {
+      generate: vi.fn().mockResolvedValue('text'),
+      queueDepth: () => 0,
+      clear: () => {},
+    };
+
+    await generateBackstory(world, entity, mockService);
+    expect(mockService.generate).not.toHaveBeenCalled();
+  });
+});

server/src/llm/backstoryGenerator.ts

@@ -0,0 +1,38 @@
+import type { Stats } from '@dflike/shared';
+import type { World } from '../ecs/World.js';
+import type { LlmService } from './llmService.js';
+
+const STAT_KEYS: (keyof Stats)[] = [
+  'strength', 'dexterity', 'constitution', 'intelligence', 'perception',
+  'sociability', 'courage', 'curiosity', 'empathy', 'temperament',
+];
+
+const STAT_ABBREVS: Record<keyof Stats, string> = {
+  strength: 'STR', dexterity: 'DEX', constitution: 'CON',
+  intelligence: 'INT', perception: 'PER',
+  sociability: 'SOC', courage: 'COU', curiosity: 'CUR',
+  empathy: 'EMP', temperament: 'TMP',
+};
+
+export function formatStatsForPrompt(stats: Stats): string {
+  return STAT_KEYS.map(k => `${STAT_ABBREVS[k]}:${Math.floor(stats[k])}`).join(', ');
+}
+
+export async function generateBackstory(
+  world: World,
+  entityId: number,
+  llmService: LlmService,
+): Promise<void> {
+  const name = world.getComponent<string>(entityId, 'name');
+  const stats = world.getComponent<Stats>(entityId, 'stats');
+  if (!name || !stats) return;
+
+  const result = await llmService.generate('backstory', {
+    npcName: name,
+    stats: formatStatsForPrompt(stats),
+  });
+
+  if (result) {
+    world.addComponent<string>(entityId, 'backstory', result);
+  }
+}