Update .env.example with all variables and simplify .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -16,11 +16,8 @@ _.log
 report.[0-9]_.[0-9]_.[0-9]_.[0-9]_.json
 
 # dotenv environment variable files
-.env
-.env.development.local
-.env.test.local
-.env.production.local
-.env.local
+.env*
+!.env.example
 
 # TanStack Router generated
 src/frontend/routeTree.gen.ts