Merge authentication into Questions API

api/QuestionsApi/Services/AuthValidation.cs

@@ -0,0 +1,30 @@
+using System.Net.Mail;
+using System.Text.RegularExpressions;
+
+namespace QuestionsApi.Services;
+
+public static partial class AuthValidation
+{
+    [GeneratedRegex("^[A-Za-z0-9_]{3,24}$", RegexOptions.CultureInvariant)]
+    private static partial Regex UsernamePattern();
+
+    public static string NormalizeEmail(string email) => email.Trim().ToLowerInvariant();
+
+    public static bool ValidEmail(string email)
+    {
+        var normalized = NormalizeEmail(email);
+        var domain = normalized.Split('@').LastOrDefault() ?? "";
+        return MailAddress.TryCreate(normalized, out _) && domain.Contains('.');
+    }
+
+    public static string? ValidateNewAccount(string username, string email, string password)
+    {
+        if (!UsernamePattern().IsMatch(username.Trim()))
+            return "Username must be 3-24 characters using only letters, numbers, or underscores.";
+        if (password.Length < 8)
+            return "Password must be at least 8 characters.";
+        if (!ValidEmail(email))
+            return "A valid email address is required.";
+        return null;
+    }
+}