Add observability for artifact storage (metrics & alerts)

[aaronsb]

From ADR-083 Phase 9 - future enhancement.

Tasks:

• Add metrics for artifact storage usage (count, size by type)
• Add metrics for localStorage cache hit/miss rates
• Add alerts for Garage storage growth

Considerations:

• Prometheus/OpenMetrics format
• Expose via /metrics endpoint
• Alert thresholds TBD based on usage

Related: ADR-083 Artifact Persistence Pattern

[aaronsb]

Implemented - artifact storage observability added