Add structured logging and centralized error handling

- Initialize slog in the serve command with terminal/file support
- Introduce `AppError` with HTTP status for unified service-layer errors
- Replace ad-hoc `api.Error` calls with `api.RespondError`
- Wrap internal errors with `model.NewInternalError` and add reference
  IDs
- Add `RequestID` middleware and switch router from `gin.Default` to
  `gin.New`

internal/server/router.go

@@ -4,11 +4,18 @@ import (
 	"github.com/gin-gonic/gin"
 
 	"github.com/dhao2001/mygo/internal/app"
+	"github.com/dhao2001/mygo/internal/middleware"
 )
 
 // NewRouter builds the Gin router and registers API routes.
 func NewRouter(webApp *app.WebApp) *gin.Engine {
-	router := gin.Default()
+	router := gin.New()
+
+	// Request ID must be first — every subsequent middleware and handler
+	// gets access to req_id in the context.
+	router.Use(middleware.RequestID())
+	router.Use(gin.Logger())
+	router.Use(gin.Recovery())
 
 	v1 := router.Group("/api/v1")