Add real-time log monitoring and system stats dashboard

Implements a comprehensive monitoring system for the admin interface:

Backend:
- New monitoring package with Redis ring buffer for log storage
- Zerolog MultiWriter to capture logs to Redis
- System stats collection (CPU, memory, disk, goroutines, GC)
- HTTP metrics middleware (request counts, latency, error rates)
- Asynq queue stats for worker process
- WebSocket endpoint for real-time log streaming
- Admin auth middleware now accepts token in query params (for WebSocket)

Frontend:
- New monitoring page with tabs (Overview, Logs, API Stats, Worker Stats)
- Real-time log viewer with level filtering and search
- System stats cards showing CPU, memory, goroutines, uptime
- HTTP endpoint statistics table
- Asynq queue depth visualization
- Enable/disable monitoring toggle in settings

Memory safeguards:
- Max 200 unique endpoints tracked
- Hourly stats reset to prevent unbounded growth
- Max 1000 log entries in ring buffer
- Max 1000 latency samples for P95 calculation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/models/subscription.go

@@ -16,6 +16,7 @@ const (
 type SubscriptionSettings struct {
 	ID                uint `gorm:"primaryKey" json:"id"`
 	EnableLimitations bool `gorm:"column:enable_limitations;default:false" json:"enable_limitations"`
+	EnableMonitoring  bool `gorm:"column:enable_monitoring;default:true" json:"enable_monitoring"`
 }
 
 // TableName returns the table name for GORM