feat(middleware): add AdminRequired authorization middleware

AdminRequired gates admin endpoints by checking user IsAdmin flag.

Placed AFTER AuthRequired; fetches user from repository, returns 403 for non-admins.

Soft-deleted users are rejected with 401 since FindByID excludes them.

Tests: admin passes, non-admin forbidden, soft-deleted admin rejected, missing user ID.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/middleware/auth_test.go

@@ -1,6 +1,8 @@
 package middleware
 
 import (
+	"context"
+	"encoding/json"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -8,8 +10,12 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
 
 	"github.com/dhao2001/mygo/internal/auth"
+	"github.com/dhao2001/mygo/internal/model"
+	"github.com/dhao2001/mygo/internal/repository"
 )
 
 func setupTestRouter(secret []byte) *gin.Engine {
@@ -196,3 +202,162 @@ func TestAuthRequiredWrongSecret(t *testing.T) {
 		t.Errorf("status = %d, want %d", rec.Code, http.StatusUnauthorized)
 	}
 }
+
+// --- AdminRequired tests ---
+
+// errorBody extracts the error.message field from a JSON response body.
+type errorBody struct {
+	Error struct {
+		Message string `json:"message"`
+	} `json:"error"`
+}
+
+func setupAdminTestDB(t *testing.T) repository.UserRepository {
+	t.Helper()
+
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	if err := db.AutoMigrate(&model.User{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+
+	return repository.NewUserRepository(db)
+}
+
+// injectUserIDMiddleware simulates AuthRequired by injecting a user_id into the context.
+func injectUserIDMiddleware(userID string) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Set(userIDKey, userID)
+		c.Next()
+	}
+}
+
+func TestAdminRequired_AdminPasses(t *testing.T) {
+	repo := setupAdminTestDB(t)
+	ctx := context.Background()
+
+	user := &model.User{
+		ID:       "admin-1",
+		Username: "admin",
+		Email:    "admin@example.com",
+		IsAdmin:  true,
+		Status:   model.StatusActive,
+	}
+	if err := repo.Create(ctx, user); err != nil {
+		t.Fatalf("Create = %v", err)
+	}
+
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(injectUserIDMiddleware("admin-1"))
+	r.Use(AdminRequired(repo))
+	r.GET("/admin", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/admin", nil)
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}
+
+func TestAdminRequired_NonAdminForbidden(t *testing.T) {
+	repo := setupAdminTestDB(t)
+	ctx := context.Background()
+
+	user := &model.User{
+		ID:       "user-1",
+		Username: "regular",
+		Email:    "regular@example.com",
+		IsAdmin:  false,
+		Status:   model.StatusActive,
+	}
+	if err := repo.Create(ctx, user); err != nil {
+		t.Fatalf("Create = %v", err)
+	}
+
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(injectUserIDMiddleware("user-1"))
+	r.Use(AdminRequired(repo))
+	r.GET("/admin", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/admin", nil)
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusForbidden)
+	}
+
+	var body errorBody
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if body.Error.Message != "admin access required" {
+		t.Errorf("message = %q, want %q", body.Error.Message, "admin access required")
+	}
+}
+
+func TestAdminRequired_SoftDeletedAdmin(t *testing.T) {
+	repo := setupAdminTestDB(t)
+	ctx := context.Background()
+
+	user := &model.User{
+		ID:       "admin-2",
+		Username: "deleted_admin",
+		Email:    "deleted_admin@example.com",
+		IsAdmin:  true,
+		Status:   model.StatusActive,
+	}
+	if err := repo.Create(ctx, user); err != nil {
+		t.Fatalf("Create = %v", err)
+	}
+
+	// Soft-delete the admin user
+	if err := repo.Delete(ctx, "admin-2"); err != nil {
+		t.Fatalf("Delete = %v", err)
+	}
+
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(injectUserIDMiddleware("admin-2"))
+	r.Use(AdminRequired(repo))
+	r.GET("/admin", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/admin", nil)
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusUnauthorized {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusUnauthorized)
+	}
+}
+
+func TestAdminRequired_NoUserID(t *testing.T) {
+	repo := setupAdminTestDB(t)
+
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(AdminRequired(repo))
+	r.GET("/admin", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"status": "ok"})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/admin", nil)
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusUnauthorized {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusUnauthorized)
+	}
+}