chore: run go fix for Go 1.26 modernization

106 files: interface{}→any, strings.Split→SplitSeq, CutPrefix/Cut, strings.Builder, slices.Contains, remove redundant // +build directives, gofmt import ordering and indentation.

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

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

backend/internal/validators/email.go

@@ -30,11 +30,11 @@ func ValidateEmail(email string) error {
 
 // NormalizeGiftCardCode strips non-alphanumeric characters and upper-cases the code.
 func NormalizeGiftCardCode(code string) string {
-	clean := ""
+	var clean strings.Builder
 	for _, char := range code {
 		if (char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z') || (char >= '0' && char <= '9') {
-			clean += string(char)
+			clean.WriteString(string(char))
 		}
 	}
-	return clean
+	return clean.String()
 }

backend/internal/validators/email_test.go

@@ -1,5 +1,4 @@
 //go:build test
-// +build test
 
 package validators