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>

service/user/list.go

@@ -2,6 +2,7 @@ package user
 
 import (
 	"nixcn-cms/data"
+	"nixcn-cms/exception"
 	"nixcn-cms/utils"
 	"strconv"
 
@@ -16,26 +17,54 @@ func List(c *gin.Context) {
 	}
 	offset, ok := c.GetQuery("offset")
 	if !ok {
-		utils.HttpResponse(c, 400, "", "offset not found")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.UserService).
+			SetEndpoint(exception.UserListEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorInvalidInput).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 
 	// Parse string to int64
 	limitNum, err := strconv.ParseInt(limit, 10, 64)
 	if err != nil {
-		utils.HttpResponse(c, 400, "", "parse string to int error")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.UserService).
+			SetEndpoint(exception.UserListEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorInvalidInput).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 	offsetNum, err := strconv.ParseInt(offset, 10, 64)
 	if err != nil {
-		utils.HttpResponse(c, 400, "", "parse string to int error")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.UserService).
+			SetEndpoint(exception.UserListEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorInvalidInput).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 
 	// Get user list from search engine
 	list, err := new(data.User).FastListUsers(limitNum, offsetNum)
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "failed list users from meilisearch")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.UserService).
+			SetEndpoint(exception.UserListEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.UserListMeilisearchFailed).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 	}
 
 	userListResp := struct {