Health check for originals storage mount

[bitbiter-dev]

Context

The API health check endpoint (GET /api/v1/healthz) currently checks the proxy storage directory (/data/proxies). The originals mount (/data/originals) is equally critical for on-demand serving of original files but has no health check.

This issue belongs to Epic 5 (API endpoints) because serving originals is an Epic 5 concern — the Worker only writes proxies, and the API serves originals on demand.

What to do

Add an OriginalsStorageHealthCheck that mirrors ProxyStorageHealthCheck:

• Healthy → /data/originals exists and is accessible
• Degraded → directory not found (mount missing; API can still serve proxies)
• Unhealthy → file system threw an exception

Register it in ServiceCollectionExtensions.AddApiHealthChecks() alongside the existing proxy check:

.AddCheck<OriginalsStorageHealthCheck>("originalsStorage");

Add the corresponding constant to AppDefaults.Storage:

internal const string OriginalsPath = "/data/originals";

Test coverage

Mirror ProxyStorageHealthCheckTests with three cases:

• Directory exists → Healthy with description
• Directory does not exist → Degraded with description
• File system throws → Unhealthy with exception

Notes

• Reuse IFileSystem (already registered as singleton for the proxy check)
• Both mounts are read-only for the API (docker-compose.yml mounts :ro)
• Degraded is appropriate for a missing mount because the API remains functional for non-originals requests