Add optional remote MongoDB replica deployment

[tdurieux]

Summary

• add primary and secondary Docker Compose configurations for an authenticated rs0 replica set
• add idempotent setup commands for a hidden, non-voting, optionally delayed secondary
• support complete MONGODB_URI connection strings while preserving standalone defaults
• document secure two-server deployment and recovery operations

Validation

• npm run build
• MongoDB connection tests (2 passing)
• shell syntax validation for replica scripts
• Docker Compose validation for primary and secondary configurations

Notes

• the two-server topology is intentionally recovery-oriented, not automatic failover
• the remote member defaults to hidden, non-voting, priority 0, and a one-hour delay
• replication complements but does not replace off-host backups