refactor: rename internal/log to internal/logging to avoid stdlib

shadowing

- refactor: rename `internal/log` → `internal/logging`, package `log` →
  `logging`
- refactor: update imports in `cmd/serve.go` and
  `internal/middleware/requestid.go`

internal/logging/logger.go

@@ -0,0 +1,81 @@
+package logging
+
+import (
+	"context"
+	"log/slog"
+	"os"
+
+	"github.com/dhao2001/mygo/internal/config"
+)
+
+// NewLogger creates a slog.Logger with dual output: terminal (stderr) and
+// optional file. Terminal and file outputs use independent log levels.
+// On file-open failure, a warning is emitted to stderr and the logger
+// continues without file output.
+func NewLogger(cfg config.LogConfig) *slog.Logger {
+	terminalLevel, _ := config.ParseLogLevel(cfg.Level)
+	fileLevel, _ := config.ParseLogLevel(cfg.FileLevel)
+
+	terminalHandler := slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
+		Level: terminalLevel,
+	})
+
+	handlers := []slog.Handler{terminalHandler}
+
+	if cfg.FilePath != "" {
+		f, err := os.OpenFile(cfg.FilePath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+		if err != nil {
+			slog.New(terminalHandler).Warn("failed to open log file, continuing without file output",
+				"path", cfg.FilePath, "error", err)
+		} else {
+			fileHandler := slog.NewTextHandler(f, &slog.HandlerOptions{
+				Level: fileLevel,
+			})
+			handlers = append(handlers, fileHandler)
+		}
+	}
+
+	handler := NewContextHandler(&multiHandler{handlers: handlers})
+	return slog.New(handler)
+}
+
+// multiHandler fans out to multiple slog.Handler implementations.
+type multiHandler struct {
+	handlers []slog.Handler
+}
+
+func (m *multiHandler) Enabled(ctx context.Context, level slog.Level) bool {
+	for _, h := range m.handlers {
+		if h.Enabled(ctx, level) {
+			return true
+		}
+	}
+	return false
+}
+
+func (m *multiHandler) Handle(ctx context.Context, r slog.Record) error {
+	for _, h := range m.handlers {
+		if h.Enabled(ctx, r.Level) {
+			if err := h.Handle(ctx, r); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (m *multiHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	clones := make([]slog.Handler, len(m.handlers))
+	for i, h := range m.handlers {
+		clones[i] = h.WithAttrs(attrs)
+	}
+	return &multiHandler{handlers: clones}
+}
+
+func (m *multiHandler) WithGroup(name string) slog.Handler {
+	clones := make([]slog.Handler, len(m.handlers))
+	for i, h := range m.handlers {
+		clones[i] = h.WithGroup(name)
+	}
+	return &multiHandler{handlers: clones}
+}