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/auth/exchange.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/url"
 	"nixcn-cms/data"
+	"nixcn-cms/exception"
 	"nixcn-cms/pkgs/authcode"
 	"nixcn-cms/utils"
 
@@ -23,38 +24,80 @@ func Exchange(c *gin.Context) {
 	err := c.ShouldBindJSON(&exchangeReq)
 	if err != nil {
 		fmt.Println(err)
-		utils.HttpResponse(c, 400, "", "invalid request")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorInvalidInput).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 
 	userIdOrig, ok := c.Get("user_id")
 	if !ok {
-		utils.HttpResponse(c, 401, "", "unauthorized")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorUnauthorized).
+			Build()
+		utils.HttpResponse(c, 401, errorCode)
 		return
 	}
 
 	userId, err := uuid.Parse(userIdOrig.(string))
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "failed to parse uuid")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorUuidParseFailed).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 		return
 	}
 
 	userData := new(data.User)
 	user, err := userData.GetByUserId(userId)
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "failed to get user id")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthExchangeGetUserIdFailed).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 		return
 	}
 
 	code, err := authcode.NewAuthCode(exchangeReq.ClientId, user.Email)
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "code gen failed")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthExchangeCodeGenFailed).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 		return
 	}
 
 	url, err := url.Parse(exchangeReq.RedirectUri)
 	if err != nil {
-		utils.HttpResponse(c, 400, "", "invalid redirect uri")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthExchangeEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthExchangeInvalidRedirectUri).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 	query := url.Query()