chore: update .gitignore to ignore all env files and retain example templates

Replace broad `.env*` ignore rule with explicit entries for real env files (`.env`, `.env.local`, `.env.development`, `.env.test`, `.env.production`) while preserving `.env.example` and adding exceptions for the new `.env.development.example`, `.env.test.example`, and `.env.production.example` templates.

Add environment-specific example files to document required variables for development, test, and production setups, including API base URLs, WebSocket endpoints, API timeouts, Auth.js secret, OAuth credentials (Google, Facebook), and Cloudflare CDN configuration.

.gitignore

@@ -31,8 +31,15 @@ yarn-error.log*
 .pnpm-debug.log*
 
 # env files (can opt-in for committing if needed)
-.env*
+.env
+.env.local
+.env.development
+.env.test
+.env.production
 !.env.example
+!.env.development.example
+!.env.test.example
+!.env.production.example
 
 # deploy archives (scripts/deploy/_deploy_lib.sh 的 archive() 生成)
 archives/