Deduplicate env examples: unify TZ usage, move update-check vars to root, align DISABLE_UPDATE_CHECK default with compose

backend/.env.example

@@ -30,8 +30,8 @@ CORS_ORIGINS=*
 ALLOWED_HOSTS=localhost,127.0.0.1
 
 # Update Check Configuration
-# Set to true to disable update checks for private repositories
-DISABLE_UPDATE_CHECK=false
-# GitHub token for private repositories (optional)
-# Get token at: https://github.com/settings/tokens (needs 'repo' scope)
-GITHUB_TOKEN=
+# Note: DISABLE_UPDATE_CHECK and GITHUB_TOKEN are managed via root .env
+# (see ../.env.example) and passed in docker-compose. Uncomment below only
+# if running backend standalone without compose.
+# DISABLE_UPDATE_CHECK=false
+# GITHUB_TOKEN=