Default new users to 'user' role instead of 'admin'

The bootstrap admin from ADMIN_USERNAME/ADMIN_PASSWORD is unaffected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/models/user.ts

@@ -71,7 +71,7 @@ export async function createUser(data: {
   passwordHash?: string | null;
 }): Promise<User> {
   const now = nowIso();
-  const role = data.role ?? "admin"; // All users are admin by default
+  const role = data.role ?? "user";
   const email = data.email.trim().toLowerCase();
 
   const [user] = await db