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/openai.go

@@ -2,6 +2,7 @@ package providers
 
 import (
 	"context"
+	"time"
 	"encoding/json"
 	"fmt"
 	"strings"