Fix: Improve Plex OAuth flow and initialization

backend/src/routes/auth.ts

@@ -8,6 +8,7 @@ const router = Router();
 
 // Plex OAuth configuration
 const PLEX_CLIENT_ID = process.env.PLEX_CLIENT_ID || '';
+const PLEX_CLIENT_SECRET = process.env.PLEX_CLIENT_SECRET || '';
 const PLEX_REDIRECT_URI = process.env.PLEX_REDIRECT_URI || '';
 const JWT_SECRET = process.env.JWT_SECRET || 'secret';
 
@@ -17,7 +18,11 @@ router.get('/plex/url', asyncHandler(async (_req, res) => {
     client_id: PLEX_CLIENT_ID,
     redirect_uri: PLEX_REDIRECT_URI,
     response_type: 'code',
-    scope: 'openid profile'
+    scope: 'openid profile',
+    'X-Plex-Product': 'CoopCoins',
+    'X-Plex-Client-Identifier': PLEX_CLIENT_ID,
+    'X-Plex-Device': 'Web Browser',
+    'X-Plex-Platform': 'Web'
   });
   
   const authUrl = `https://app.plex.tv/auth#?${params.toString()}`;
@@ -39,6 +44,7 @@ router.post('/plex/callback', asyncHandler(async (req, res) => {
     {
       code,
       client_id: PLEX_CLIENT_ID,
+      client_secret: PLEX_CLIENT_SECRET,
       grant_type: 'authorization_code'
     },
     {