feat: add .env.example with required environment variables

.env.example

@@ -0,0 +1,6 @@
+DATABASE_URL=postgresql://postgres:postgres@localhost:5432/comparaison
+BETTER_AUTH_SECRET=change-me-to-random-string
+OPENAI_API_KEY=
+PERPLEXITY_API_KEY=
+TAVILY_API_KEY=
+NEXT_PUBLIC_APP_URL=http://localhost:3000

.gitignore

@@ -32,6 +32,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+!.env.example
 
 # vercel
 .vercel