chore: add .env files to gitignore

Add .env files to gitignore to prevent accidentally committing
sensitive credentials like database passwords, JWT secrets, and
API keys to version control.

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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

.gitignore

@@ -1 +1,7 @@
+# Kubernetes secrets
 deploy/k8s/secrets.yaml
+
+# Environment variables
+.env
+backend/.env
+frontend/.env