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

Beginne damit, das zugehörige Issue coder/nexus#256 und das interne Issue #1106 zu lesen, und verfolge anschließend den in diesen Issues referenzierten bestehenden Pfad zur Nutzungsberichterstattung. Abgeschlossen sein sollte die Arbeit mit einem regelmäßigen, ressourcenschonenden Heartbeat von Deployments sowie einer Möglichkeit, anhand seines Empfangs Verbindungs- oder Firewall-Probleme von ausbleibender Nutzung zu unterscheiden.

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