Convert MongoDB from standalone to replica set

[Copilot]

MongoDB standalone mode does not support multi-document transactions, which are required for atomic merge operations across FrontierCollection and MergeCollection. This converts to a single-member replica set (rs0) in both development and Docker/Kubernetes environments.

Development (npm start)

• scripts/startDatabase.js (new): Replaces predatabase+database npm scripts and the now-removed setupMongo.js. Spawns mongod --replSet rs0, polls until ready, calls rs.initiate(), then stays alive until mongod exits. Failures are fatal: SIGINT/SIGTERM handlers are registered early, startup/init failures call process.exit(1), and mongod's exit code is propagated via process.exit(exitCode ?? 1).
• scripts/setupMongo.js (removed): No longer needed; its directory-creation logic is handled by startDatabase.js.
• package.json: database script → node scripts/startDatabase.js; predatabase removed.
• .gitignore: Replaced setupMongo.js exception with startDatabase.js exception.

Docker / Kubernetes

• database/init/00-replica-set.js (new): Placed in /docker-entrypoint-initdb.d/, runs on first container startup. Calls rs.initiate() with a configurable advertised hostname via MONGO_INITDB_REPLICA_HOST (defaults to localhost:27017). Waits up to 30s for PRIMARY election before allowing subsequent init scripts to run.
• database.yaml (Kubernetes Helm): Adds args: ["--replSet", "rs0"] and sets MONGO_INITDB_REPLICA_HOST to the pod IP so the replica set member hostname is tracked per pod. The postStart lifecycle hook initializes the replica set on first start and reconfigures the member host (rs.reconfig) when the pod IP changes after a restart, keeping the replica set config in sync.

Configuration

• appsettings.json: Connection strings now explicitly include ?replicaSet=rs0.
• .vscode/tasks.json: run-mongo task updated to invoke npm run database (which runs scripts/startDatabase.js) instead of calling mongod directly, ensuring replica set initialization is performed automatically on first run.
• README.md: MongoDB Compass connection string updated to mongodb://localhost:27017/?replicaSet=rs0.

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This change is 

Summary by CodeRabbit

• New Features

  • Enabled MongoDB replica set (rs0) support for both local development and Kubernetes deployments.
  • Improved database initialization with automatic replica set configuration and validation.

• Documentation

  • Updated MongoDB connection instructions and database maintenance commands with replica set configuration.