feat: add interval-based metrics snapshotting

[d3v-active]

Closes #148

This PR adds support for taking periodic snapshots of metrics on a fixed interval, independent of read traffic.

Changes Included:

• Configuration: Added the METRICS_SNAPSHOT_INTERVAL_MS environment variable to configure the snapshot interval. By default, this is disabled to preserve previous behavior.
• Metrics Router: Updated the metricsRouter factory (src/routes/metrics.ts) to accept an optional snapshotIntervalMs parameter. When configured, it kicks off a setInterval that records snapshots autonomously.
• Graceful Shutdown: The interval timer is properly unref()'d to ensure it does not block the Node.js process from exiting gracefully.
• Backward Compatibility: Read-triggered snapshots via GET / are fully preserved. This means manual polling will still capture the exact state upon read alongside the interval-based snapshots.
• Testing: Added test coverage in src/routes/metrics.test.ts utilizing jest.useFakeTimers() to verify that the interval-driven snapshots are recorded accurately independent of any API calls.