Keep MongoDB off the Internet and its password out of git

MongoDB was published on 0.0.0.0:27017 with the default "example" root
password. It now listens on 127.0.0.1 only (the apps use the Docker
network), and the root password comes from MONGO_PASSWORD in a
git-ignored .env file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -15,3 +15,6 @@ static/device_snapshots/
 
 # Config locale spécifique à cette machine
 .claude/
+
+# Mot de passe MongoDB (variables de docker-compose)
+.env