Features:
- User cache tracks radio IDs and last heard repeater for routing optimization
- 10-minute timeout (configurable via user_cache.timeout in config)
- Automatic cleanup every 60s to prevent memory growth
- Dashboard Last Heard display (top 10 users)
- View Full Cache modal showing up to 50 users
- Only displays talker alias when available (shows '-' otherwise)
- Periodic broadcast to dashboard every 10s
Logging improvements:
- Commented out per-packet validation debug logs
- Reduced dashboard logging (disabled access logs, WebSocket logs to DEBUG)
- All high-frequency debug logs marked with 'Per-packet logging' for easy re-enabling
Tests:
- Comprehensive test suite for user cache functionality
- All tests passing
