Add heartbeat for usage reporting for Tallyman/Metronome

## Problem
We currently have no way to know if a deployment can reach our usage reporting system until they actually send usage data. This makes it difficult to distinguish between customers blocked by firewalls versus customers with low/no usage.

## Solution
Add a heartbeat mechanism where deployments send a periodic ping to indicate they can reach the usage reporting system.

- Send lightweight payload (e.g., "I can reach you") on a regular interval
- Use this data to identify firewall issues during POC and post-deployment
- Helps CS team proactively identify connectivity problems

## Related Issues

- [x] https://github.com/coder/nexus/issues/256
- [ ] https://github.com/coder/internal/issues/1106