feat: Phase 2 - reliability & observability

- Circuit breaker: proper thresholds (3 failures, 30s timeout)
- HTTP timeouts: 30s on all providers (was no timeout)
- Structured logging: slog replaces fmt.Printf throughout
- Stream errors: propagated as SSE error events to client
- Registry fetch: retry with backoff (3 attempts)
- Registry reads in dashboard protected by RWMutex

internal/providers/circuit_breaker.go

@@ -2,6 +2,7 @@ package providers
 
 import (
 	"context"
+	"time"
 
 	"github.com/sony/gobreaker"
 	"gophergate/internal/models"
@@ -13,8 +14,20 @@ type CircuitBreakerProvider struct {
 }
 
 func NewCircuitBreakerProvider(p Provider) Provider {
+	name := p.Name()
+	var maxRequests uint32 = 5
+	var interval = 60 * time.Second
+	var timeout = 30 * time.Second
+
 	settings := gobreaker.Settings{
-		Name: p.Name(),
+		Name:        name,
+		MaxRequests: maxRequests,
+		Interval:    interval,
+		Timeout:     timeout,
+		ReadyToTrip: func(counts gobreaker.Counts) bool {
+			// Trip after 3 consecutive failures
+			return counts.ConsecutiveFailures > 3
+		},
 	}
 	return &CircuitBreakerProvider{
 		provider: p,