chore: add COOKIE_SECRET to .env, .env.example, .env.sample

Generated a 64-char hex secret (openssl rand -hex 32 equivalent) and
added it to .env. Updated .env.example and .env.sample with the new
required variable and a generation hint. This is the production secret
for HMAC-signing the emby_user session cookie.

.env.example

@@ -2,6 +2,10 @@
 PORT=3001
 LOG_LEVEL=info
 
+# Cookie signing secret for tamper-proof session cookies
+# Required in production. Generate with: openssl rand -hex 32
+COOKIE_SECRET=your_cookie_secret_here
+
 # Background polling interval in ms (default: 5000)
 # Set to 0 or "off" to disable and fetch on-demand instead
 # POLL_INTERVAL=5000