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Wino.Mail.Contracts/Common/ApiEnvelope.cs

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+namespace Wino.Mail.Api.Contracts.Common;
+
+public sealed class ApiEnvelope<T>
+{
+    public bool IsSuccess { get; init; }
+    public string? ErrorCode { get; init; }
+    public T? Result { get; init; }
+    public QuotaInfoDto? Quota { get; init; }
+
+    public static ApiEnvelope<T> Success(T result, QuotaInfoDto? quota = null)
+        => new()
+        {
+            IsSuccess = true,
+            Result = result,
+            Quota = quota,
+        };
+
+    public static ApiEnvelope<T> Failure(string errorCode, QuotaInfoDto? quota = null)
+        => new()
+        {
+            IsSuccess = false,
+            ErrorCode = errorCode,
+            Quota = quota,
+        };
+}

Wino.Mail.Contracts/Common/ApiErrorCodes.cs

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+namespace Wino.Mail.Api.Contracts.Common;
+
+public static class ApiErrorCodes
+{
+    public const string InvalidCredentials = "INVALID_CREDENTIALS";
+    public const string AccountLocked = "ACCOUNT_LOCKED";
+    public const string AccountBanned = "ACCOUNT_BANNED";
+    public const string AccountSuspended = "ACCOUNT_SUSPENDED";
+    public const string RefreshTokenInvalid = "REFRESH_TOKEN_INVALID";
+    public const string EmailAlreadyRegistered = "EMAIL_ALREADY_REGISTERED";
+    public const string ExternalLoginEmailRequired = "EXTERNAL_LOGIN_EMAIL_REQUIRED";
+    public const string ExternalLoginInvalid = "EXTERNAL_LOGIN_INVALID";
+    public const string ExternalAuthStateInvalid = "EXTERNAL_AUTH_STATE_INVALID";
+    public const string ExternalAuthCodeInvalid = "EXTERNAL_AUTH_CODE_INVALID";
+    public const string AiPackRequired = "AI_PACK_REQUIRED";
+    public const string AiQuotaExceeded = "AI_QUOTA_EXCEEDED";
+    public const string AiHtmlEmpty = "AI_HTML_EMPTY";
+    public const string AiHtmlTooLarge = "AI_HTML_TOO_LARGE";
+    public const string AiUnsupportedLanguage = "AI_UNSUPPORTED_LANGUAGE";
+    public const string AiSanitizationFailed = "AI_SANITIZATION_FAILED";
+    public const string AiProviderUnavailable = "AI_PROVIDER_UNAVAILABLE";
+    public const string AiRequestBlocked = "AI_REQUEST_BLOCKED";
+    public const string AiInternalError = "AI_INTERNAL_ERROR";
+    public const string PaddleWebhookInvalid = "PADDLE_WEBHOOK_INVALID";
+    public const string Forbidden = "FORBIDDEN";
+    public const string ValidationFailed = "VALIDATION_FAILED";
+}

Wino.Mail.Contracts/Common/QuotaInfoDto.cs

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+namespace Wino.Mail.Api.Contracts.Common;
+
+public sealed record QuotaInfoDto(
+    bool HasAiPack,
+    string EntitlementStatus,
+    DateTimeOffset? CurrentPeriodStartUtc,
+    DateTimeOffset? CurrentPeriodEndUtc,
+    int? MonthlyLimit,
+    int? Used,
+    int? Remaining);