fix(auth): coerce Plex ID to string for Prisma

Plex API returns user.id as integer but Prisma schema expects
String for plexId. Wrapped with String() to fix login 500.

backend/src/routes/auth.ts

@@ -1,8 +1,8 @@
-import { Router } from "express";
 import axios from "axios";
+import { Router } from "express";
 import jwt from "jsonwebtoken";
-import { prisma } from "../utils/prisma";
 import { asyncHandler } from "../middleware/errorHandler";
+import { prisma } from "../utils/prisma";
 
 const router = Router();
 
@@ -86,13 +86,13 @@ router.post(
 
 		// Find or create user
 		let user = await prisma.user.findUnique({
-			where: { plexId: plexUser.id },
+			where: { plexId: String(plexUser.id) },
 		});
 
 		if (!user) {
 			user = await prisma.user.create({
 				data: {
-					plexId: plexUser.id,
+					plexId: String(plexUser.id),
 					plexUsername: plexUser.username || plexUser.email,
 					email: plexUser.email,
 					isAdmin: false,