feat: update environment variables for admin user and adjust signup username pattern

.env.example

@@ -20,6 +20,12 @@ PRICE_UPDATE_INTERVAL_MINUTES=60
 # Max pagination pages to fetch when counting posts (default: 5 = up to 200 posts)
 MAX_PAGES_PER_HASHTAG=5
 
+# Initial admin user — only used by `npm run db:seed`, not the running app.
+# Pass these at seed time: docker exec -e ADMIN_USERNAME=x -e ADMIN_PASSWORD=y <container> npm run db:seed
+# If omitted, defaults to admin / changeme123 (change immediately after first login).
+# ADMIN_USERNAME=admin
+# ADMIN_PASSWORD=changeme123
+
 # Postgres (used by postgres container in prod-compose.yml)
 POSTGRES_DB=hashex
 POSTGRES_USER=hashex