fix(security #13,#14): revoke Emby token on logout; stable DeviceId prevents unbounded sessions

#13 Logout doesn't revoke Emby token:
  - Added in-memory tokenStore (userId -> { accessToken })
  - AccessToken stored server-side after successful login; never sent
    to client
  - POST /logout calls Emby POST /Sessions/Logout with the stored
    token before clearing it; failure is warned but does not block
    the local cookie clear

#14 Unbounded Emby session creation per login:
  - DeviceId in the Emby auth request is now a stable SHA-256 hash
    of the lowercase username (sofarr-<16 hex chars>)
  - Emby treats the same DeviceId as the same device and reuses the
    existing session slot instead of creating a new one each login

server/routes/auth.js

@@ -1,8 +1,28 @@
 const express = require('express');
 const axios = require('axios');
+const crypto = require('crypto');
 const rateLimit = require('express-rate-limit');
 const router = express.Router();
 
+const EMBY_URL = process.env.EMBY_URL;
+
+// Server-side token store: userId -> { accessToken }
+// Keeps AccessToken off the client; required for logout revocation.
+const tokenStore = new Map();
+
+function storeToken(userId, accessToken) {
+  tokenStore.set(userId, { accessToken });
+}
+
+function getToken(userId) {
+  return tokenStore.get(userId) || null;
+}
+
+function clearToken(userId) {
+  tokenStore.delete(userId);
+}
+
+
 const loginLimiter = rateLimit({
   windowMs: 15 * 60 * 1000, // 15 minutes
   max: 10,
@@ -18,13 +38,16 @@ router.post('/login', loginLimiter, async (req, res) => {
     
     console.log(`[Auth] Attempting login for user: ${username}`);
     
-    // Authenticate with Emby
+    // Authenticate with Emby using a stable DeviceId derived from the username.
+    // Using a deterministic DeviceId causes Emby to reuse the existing session
+    // for this device rather than creating a new one on each login.
+    const stableDeviceId = 'sofarr-' + crypto.createHash('sha256').update(username.toLowerCase()).digest('hex').slice(0, 16);
     const authResponse = await axios.post(`${EMBY_URL}/Users/authenticatebyname`, {
       Username: username,
       Pw: password
     }, {
       headers: {
-        'X-Emby-Authorization': `MediaBrowser Client="MediaDashboard", Device="Browser", DeviceId="dashboard-${Date.now()}", Version="1.0.0"`
+        'X-Emby-Authorization': `MediaBrowser Client="sofarr", Device="sofarr", DeviceId="${stableDeviceId}", Version="1.0.0"`
       }
     });
     
@@ -38,12 +61,13 @@ router.post('/login', loginLimiter, async (req, res) => {
     });
     
     const user = userResponse.data;
-    console.log(`[Auth] Login successful for user: ${user.Name}, isAdmin: ${!!(user.Policy && user.Policy.IsAdministrator)}`);
-    
-    // Set authentication cookie.
-    // Note: token is intentionally excluded from the cookie — it is not needed client-side.
-    // Cookie is signed when COOKIE_SECRET is set (recommended in production).
     const isAdmin = !!(user.Policy && user.Policy.IsAdministrator);
+    console.log(`[Auth] Login successful for user: ${user.Name}, isAdmin: ${isAdmin}`);
+
+    // Store token server-side; it is never sent to the client.
+    storeToken(user.Id, authData.AccessToken);
+    
+    // Set authentication cookie (signed when COOKIE_SECRET is set).
     const cookiePayload = JSON.stringify({ id: user.Id, name: user.Name, isAdmin });
     const signed = !!process.env.COOKIE_SECRET;
     res.cookie('emby_user', cookiePayload, {
@@ -56,11 +80,7 @@ router.post('/login', loginLimiter, async (req, res) => {
     
     res.json({
       success: true,
-      user: {
-        id: user.Id,
-        name: user.Name,
-        isAdmin: isAdmin
-      }
+      user: { id: user.Id, name: user.Name, isAdmin }
     });
   } catch (error) {
     console.error(`[Auth] Login failed:`, error.message);
@@ -98,7 +118,22 @@ router.get('/me', (req, res) => {
 });
 
 // Logout
-router.post('/logout', (req, res) => {
+router.post('/logout', async (req, res) => {
+  const user = parseSessionCookie(req);
+  if (user) {
+    const stored = getToken(user.id);
+    if (stored) {
+      try {
+        await axios.post(`${EMBY_URL}/Sessions/Logout`, {}, {
+          headers: { 'X-MediaBrowser-Token': stored.accessToken }
+        });
+        console.log(`[Auth] Revoked Emby token for user: ${user.name}`);
+      } catch (err) {
+        console.warn(`[Auth] Failed to revoke Emby token for ${user.name}:`, err.message);
+      }
+      clearToken(user.id);
+    }
+  }
   res.clearCookie('emby_user', {
     httpOnly: true,
     secure: process.env.NODE_ENV === 'production',