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/logger/logger.go

@@ -0,0 +1,47 @@
+package logger
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"strings"
+)
+
+var level = slog.LevelInfo
+
+func init() {
+	env := os.Getenv("LLM_PROXY_LOG_LEVEL")
+	switch strings.ToLower(env) {
+	case "debug":
+		level = slog.LevelDebug
+	case "warn":
+		level = slog.LevelWarn
+	case "error":
+		level = slog.LevelError
+	}
+
+	h := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+		Level: level,
+	})
+	slog.SetDefault(slog.New(h))
+}
+
+// Warn is a helper to emit structured warnings.
+func Warn(msg string, args ...any) {
+	slog.Warn(msg, args...)
+}
+
+// Error is a helper to emit structured errors.
+func Error(msg string, args ...any) {
+	slog.Error(msg, args...)
+}
+
+// Debug is a helper to emit structured debug messages.
+func Debug(msg string, args ...any) {
+	slog.Debug(msg, args...)
+}
+
+// Ctx wraps slog with context.
+func Ctx(ctx context.Context) *slog.Logger {
+	return slog.Default()
+}