refactor: standardize error handling with exception.Builder

- Replace hardcoded error messages with structured error codes using exception.Builder.
- Introduce new common error constants in exception/common.go (CommonErrorInvalidInput, CommonErrorUserNotFound, etc.).
- Update exception/specific.go with domain-specific errors and remove redundant ones.
- Apply consistent error handling across auth, event, user services and middleware.

Co-authored-by: Gemini <gemini@google.com>
Signed-off-by: Noa Virellia <noa@requiem.garden>

middleware/permission.go

@@ -2,6 +2,7 @@ package middleware
 
 import (
 	"nixcn-cms/data"
+	"nixcn-cms/exception"
 	"nixcn-cms/utils"
 
 	"github.com/gin-gonic/gin"
@@ -15,19 +16,40 @@ func Permission(requiredLevel uint) gin.HandlerFunc {
 		if !ok {
 			userIdOrig, ok := c.Get("user_id")
 			if !ok || userIdOrig.(string) == "" {
-				utils.HttpAbort(c, 401, "", "missing user id")
+				errorCode := new(exception.Builder).
+					SetStatus(exception.ErrorStatusUser).
+					SetService(exception.MiddlewarePermissionService).
+					SetEndpoint(exception.MiddlewareEndpoint).
+					SetType(exception.ErrorTypeCommon).
+					SetOriginal(exception.CommonErrorMissingUserId).
+					Build()
+				utils.HttpAbort(c, 401, errorCode)
 				return
 			}
 
 			userId, err := uuid.Parse(userIdOrig.(string))
 			if err != nil {
-				utils.HttpAbort(c, 500, "", "error parsing user id")
+				errorCode := new(exception.Builder).
+					SetStatus(exception.ErrorStatusServer).
+					SetService(exception.MiddlewarePermissionService).
+					SetEndpoint(exception.MiddlewareEndpoint).
+					SetType(exception.ErrorTypeCommon).
+					SetOriginal(exception.CommonErrorUuidParseFailed).
+					Build()
+				utils.HttpAbort(c, 500, errorCode)
 				return
 			}
 
 			userData, err := new(data.User).GetByUserId(userId)
 			if err != nil {
-				utils.HttpAbort(c, 404, "", "user not found")
+				errorCode := new(exception.Builder).
+					SetStatus(exception.ErrorStatusUser).
+					SetService(exception.MiddlewarePermissionService).
+					SetEndpoint(exception.MiddlewareEndpoint).
+					SetType(exception.ErrorTypeCommon).
+					SetOriginal(exception.CommonErrorUserNotFound).
+					Build()
+				utils.HttpAbort(c, 404, errorCode)
 				return
 			}
 
@@ -38,7 +60,14 @@ func Permission(requiredLevel uint) gin.HandlerFunc {
 		}
 
 		if permissionLevel < requiredLevel {
-			utils.HttpAbort(c, 403, "", "permission denied")
+			errorCode := new(exception.Builder).
+				SetStatus(exception.ErrorStatusUser).
+				SetService(exception.MiddlewarePermissionService).
+				SetEndpoint(exception.MiddlewareEndpoint).
+				SetType(exception.ErrorTypeCommon).
+				SetOriginal(exception.CommonErrorPermissionDenied).
+				Build()
+			utils.HttpAbort(c, 403, errorCode)
 			return
 		}