Refactor handlers to use MustGetUserID

- Fix: replace repeated authorization checks in handlers with the new
  MustGetUserID helper, which panics if the user ID is missing. This
  simplifies handler code by eliminating redundant error handling.
- Tests: update auth tests to verify the panic behavior in unprotected
  routes.

internal/middleware/auth_test.go

@@ -17,7 +17,7 @@ func setupTestRouter(secret []byte) *gin.Engine {
 	r := gin.New()
 	r.Use(AuthRequired(secret))
 	r.GET("/protected", func(c *gin.Context) {
-		c.JSON(http.StatusOK, gin.H{"user_id": GetUserID(c)})
+		c.JSON(http.StatusOK, gin.H{"user_id": MustGetUserID(c)})
 	})
 	return r
 }
@@ -137,6 +137,47 @@ func TestGetUserID(t *testing.T) {
 	}
 }
 
+func TestMustGetUserID(t *testing.T) {
+	secret := []byte("test-secret")
+	token, err := auth.GenerateAccessToken("bob-99", secret, 15*time.Minute)
+	if err != nil {
+		t.Fatalf("GenerateAccessToken = %v", err)
+	}
+
+	r := setupTestRouter(secret)
+	req := httptest.NewRequest(http.MethodGet, "/protected", nil)
+	req.Header.Set("Authorization", "Bearer "+token)
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d", rec.Code)
+	}
+
+	body := rec.Body.String()
+	if !strings.Contains(body, "bob-99") {
+		t.Errorf("response body %q does not contain user id", body)
+	}
+}
+
+func TestMustGetUserIDPanics(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.GET("/naked", func(c *gin.Context) {
+		MustGetUserID(c) // should panic — no AuthRequired middleware applied
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/naked", nil)
+	rec := httptest.NewRecorder()
+
+	defer func() {
+		if r := recover(); r == nil {
+			t.Error("expected panic, got none")
+		}
+	}()
+	r.ServeHTTP(rec, req)
+}
+
 func TestAuthRequiredWrongSecret(t *testing.T) {
 	secret := []byte("test-secret")
 	token, err := auth.GenerateAccessToken("user-1", secret, 15*time.Minute)