chore: add .env.example template and clean up deploy messages

Introduce a tracked .env.example file documenting all required environment
variables (app env, API/WebSocket base URLs, timeouts, Google/Facebook OAuth
credentials, Cloudflare CDN zone/token). Update .gitignore to allow this
file despite the blanket .env* ignore rule. Also simplify the push
success/failure output in scripts/deploy/_deploy_lib.sh by removing
verbose troubleshooting hints.

.gitignore

@@ -32,6 +32,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+!.env.example
 
 # deploy archives (scripts/deploy/_deploy_lib.sh 的 archive() 生成)
 archives/