feat: add storage stats and average latency metrics

Track and display storage and performance metrics that were previously
showing zeros.

Storage metrics:
- Add GetStats() method to storage returning event count and DB size
- Store database file path for size calculation
- Update metrics every 30 seconds via goroutine in main.go
- Display event count and DB size (MB) in dashboard

Performance metrics:
- Calculate average latency from histogram sum/count metrics
- Display as milliseconds in dashboard
- Formula: (duration_sum / duration_count) * 1000

Missing metrics (deferred):
- Connections: requires connection lifecycle tracking (gRPC + WebSocket)
- Deletions: count would need API change to ProcessDeletion

Dashboard now shows accurate storage stats and request latency!

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