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/magic.go

@@ -2,6 +2,7 @@ package auth
 
 import (
 	"net/url"
+	"nixcn-cms/exception"
 	"nixcn-cms/pkgs/authcode"
 	"nixcn-cms/pkgs/email"
 	"nixcn-cms/pkgs/turnstile"
@@ -23,27 +24,55 @@ func Magic(c *gin.Context) {
 	// Parse request
 	var req MagicRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
-		utils.HttpResponse(c, 400, "", "invalid request")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthMagicEndpoint).
+			SetType(exception.ErrorTypeCommon).
+			SetOriginal(exception.CommonErrorInvalidInput).
+			Build()
+		utils.HttpResponse(c, 400, errorCode)
 		return
 	}
 
 	// Cloudflare turnstile
 	ok, err := turnstile.VerifyTurnstile(req.TurnstileToken, c.ClientIP())
 	if err != nil || !ok {
-		utils.HttpResponse(c, 403, "", "turnstile failed")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusClient).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthMagicEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthMagicTurnstileFailed).
+			Build()
+		utils.HttpResponse(c, 403, errorCode)
 		return
 	}
 
 	code, err := authcode.NewAuthCode(req.ClientId, req.Email)
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "code gen failed")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthMagicEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthMagicCodeGenFailed).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 		return
 	}
 
 	externalUrl := viper.GetString("server.external_url")
 	url, err := url.Parse(externalUrl)
 	if err != nil {
-		utils.HttpResponse(c, 500, "", "invalid external url")
+		errorCode := new(exception.Builder).
+			SetStatus(exception.ErrorStatusServer).
+			SetService(exception.AuthService).
+			SetEndpoint(exception.AuthMagicEndpoint).
+			SetType(exception.ErrorTypeSpecific).
+			SetOriginal(exception.AuthMagicInvalidExternalUrl).
+			Build()
+		utils.HttpResponse(c, 500, errorCode)
 		return
 	}
 
@@ -66,7 +95,14 @@ func Magic(c *gin.Context) {
 		// Send email using resend
 		emailClient, err := new(email.Client).NewSMTPClient()
 		if err != nil {
-			utils.HttpResponse(c, 500, "", "invalid email config")
+			errorCode := new(exception.Builder).
+				SetStatus(exception.ErrorStatusServer).
+				SetService(exception.AuthService).
+				SetEndpoint(exception.AuthMagicEndpoint).
+				SetType(exception.ErrorTypeSpecific).
+				SetOriginal(exception.AuthMagicInvalidEmailConfig).
+				Build()
+			utils.HttpResponse(c, 500, errorCode)
 			return
 		}
 		emailClient.Send(