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/utils/registry.go

@@ -6,38 +6,48 @@ import (
 	"log"
 	"time"
 
-	"gophergate/internal/models"
 	"github.com/go-resty/resty/v2"
+	"gophergate/internal/models"
 )
 
 const ModelsDevURL = "https://models.dev/api.json"
 
 func FetchRegistry() (*models.ModelRegistry, error) {
-	log.Printf("Fetching model registry from %s", ModelsDevURL)
-
 	client := resty.New().SetTimeout(10 * time.Second)
-	resp, err := client.R().Get(ModelsDevURL)
-	if err != nil {
-		return nil, fmt.Errorf("failed to fetch registry: %w", err)
+
+	var lastErr error
+	for attempt := 0; attempt < 3; attempt++ {
+		if attempt > 0 {
+			backoff := time.Duration(1<<attempt) * time.Second
+			time.Sleep(backoff)
+		}
+
+		resp, err := client.R().Get(ModelsDevURL)
+		if err != nil {
+			lastErr = fmt.Errorf("attempt %d: %w", attempt+1, err)
+			continue
+		}
+		if !resp.IsSuccess() {
+			lastErr = fmt.Errorf("attempt %d: HTTP %d", attempt+1, resp.StatusCode())
+			continue
+		}
+
+		var providers map[string]models.ProviderInfo
+		if err := json.Unmarshal(resp.Body(), &providers); err != nil {
+			lastErr = fmt.Errorf("attempt %d: unmarshal: %w", attempt+1, err)
+			continue
+		}
+
+		log.Println("Successfully loaded model registry")
+		return &models.ModelRegistry{Providers: providers}, nil
 	}
 
-	if !resp.IsSuccess() {
-		return nil, fmt.Errorf("failed to fetch registry: HTTP %d", resp.StatusCode())
-	}
-
-	var providers map[string]models.ProviderInfo
-	if err := json.Unmarshal(resp.Body(), &providers); err != nil {
-		return nil, fmt.Errorf("failed to unmarshal registry: %w", err)
-	}
-
-	log.Println("Successfully loaded model registry")
-	return &models.ModelRegistry{Providers: providers}, nil
+	return nil, fmt.Errorf("failed to fetch registry after 3 attempts: %w", lastErr)
 }
 
 func CalculateCost(registry *models.ModelRegistry, modelID string, promptTokens, completionTokens, reasoningTokens, cacheRead, cacheWrite uint32) float64 {
 	meta := registry.FindModel(modelID)
 	if meta == nil || meta.Cost == nil {
-		log.Printf("[DEBUG] CalculateCost: model %s not found or has no cost metadata", modelID)
 		return 0.0
 	}
 
@@ -62,8 +72,5 @@ func CalculateCost(registry *models.ModelRegistry, modelID string, promptTokens,
 		cost += float64(cacheWrite) * (*meta.Cost.CacheWrite) / 1000000.0
 	}
 
-	log.Printf("[DEBUG] CalculateCost: model=%s, uncached=%d, completion=%d, reasoning=%d, cache_read=%d, cache_write=%d, cost=%f (input_rate=%f, output_rate=%f)", 
-		modelID, uncachedTokens, completionTokens, reasoningTokens, cacheRead, cacheWrite, cost, meta.Cost.Input, meta.Cost.Output)
-
 	return cost
 }