Add .env files to gitignore and update README with setup instructions

- Add .env.dev and .env.prod to .gitignore
- Document environment variable configuration in README
- Include instructions for generating secure keys

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -130,6 +130,8 @@ celerybeat.pid
 
 # Environments
 .env
+.env.dev
+.env.prod
 .venv
 env/
 venv/