fix: move API keys to .env.local, remove localStorage dual system

Single source of truth: OPENAI_API_KEY in .env.local. Deleted the
apiKeyService class/singleton that stored keys in localStorage.
All server code reads process.env directly. FirstRunModal and
Settings now show .env.local instructions instead of key input.

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

app/api/health/route.ts

@@ -5,7 +5,7 @@
 
 import { NextResponse } from 'next/server';
 import { checkDatabaseHealth } from '@/services/database';
-import { hasValidOpenAiKey } from '@/services/database/descriptionService';
+import { hasValidOpenAiKey } from '@/services/storage/apiKeys';
 
 export const runtime = 'nodejs';