feat: Auto-seed admin user on container start

- prestart.sh: runs seed_admin.py after migrations
- seed_admin.py: reads ADMIN_EMAIL and ADMIN_PASSWORD from env vars
- Creates default tenant + admin role + admin user if not exists

prestart.sh

@@ -100,6 +100,9 @@ asyncio.run(set_password())
 "
 fi
 
+echo "[prestart] Seeding admin user if not exists..."
+python3 /app/scripts/seed_admin.py || echo "[prestart] WARNING: Admin seed failed (may already exist)"
+
 echo "[prestart] Starting uvicorn on 0.0.0.0:8000 (workers=1)..."
 exec uvicorn app.main:app \
     --host 0.0.0.0 \