feat: add message content length validation

- Backend: MinMessageLength=10, MaxMessageLength=5000
- Frontend: Real-time character counter
- Frontend: Client-side validation before submission
- Frontend: Error messages for length violations
- Applied to both Submit and Update operations

Prevents empty or excessively long messages while maintaining
user-friendly feedback on character count.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Application/Services/InquiryService.cs

@@ -18,6 +18,9 @@ public class InquiryService(
     private static readonly Regex PhoneRegex = new(
         @"^(0(?:2|3[1-3]|4[1-4]|5[1-5]|6[1-4]|70|50[5-9]|[7-9](?:\d{1,2})?)\d{7,8}|0\d{9,10})$");
 
+    private const int MinMessageLength = 10;
+    private const int MaxMessageLength = 5000;
+
     public async Task<int> SubmitAsync(
         string name, string phone, string serviceType, string message,
         string? email = null, string? ipAddress = null, bool suppressNotification = false, CancellationToken ct = default)
@@ -34,13 +37,20 @@ public class InquiryService(
         if (string.IsNullOrWhiteSpace(message))
             throw new ValidationException("문의 내용을 입력하세요.");
 
+        var trimmedMessage = message.Trim();
+        if (trimmedMessage.Length < MinMessageLength)
+            throw new ValidationException($"문의 내용은 최소 {MinMessageLength}자 이상이어야 합니다.");
+
+        if (trimmedMessage.Length > MaxMessageLength)
+            throw new ValidationException($"문의 내용은 최대 {MaxMessageLength}자 이하여야 합니다.");
+
         var inquiry = new Inquiry
         {
             Name = name.Trim(),
             Phone = formattedPhone,
             Email = string.IsNullOrWhiteSpace(email) ? null : email.Trim(),
             ServiceType = serviceType ?? "기타",
-            Message = message.Trim(),
+            Message = trimmedMessage,
             IpAddress = ipAddress,
             Status = InquiryStatusMapper.ToStorageValue(InquiryStatus.New),
             CreatedAt = DateTime.UtcNow
@@ -96,6 +106,13 @@ public class InquiryService(
         if (string.IsNullOrWhiteSpace(dto.Message))
             throw new ValidationException("문의 내용을 입력하세요.");
 
+        var trimmedUpdateMessage = dto.Message.Trim();
+        if (trimmedUpdateMessage.Length < MinMessageLength)
+            throw new ValidationException($"문의 내용은 최소 {MinMessageLength}자 이상이어야 합니다.");
+
+        if (trimmedUpdateMessage.Length > MaxMessageLength)
+            throw new ValidationException($"문의 내용은 최대 {MaxMessageLength}자 이하여야 합니다.");
+
         if (!InquiryStatusMapper.TryParse(dto.Status, out var parsedStatus))
             throw new ValidationException("지원하지 않는 문의 상태입니다.");
 
@@ -103,7 +120,7 @@ public class InquiryService(
         inquiry.Phone = FormatPhoneNumber(cleanPhone);
         inquiry.Email = string.IsNullOrWhiteSpace(dto.Email) ? null : dto.Email.Trim();
         inquiry.ServiceType = string.IsNullOrWhiteSpace(dto.ServiceType) ? "기타" : dto.ServiceType.Trim();
-        inquiry.Message = dto.Message.Trim();
+        inquiry.Message = trimmedUpdateMessage;
         inquiry.Status = InquiryStatusMapper.ToStorageValue(parsedStatus);
         inquiry.AdminMemo = dto.AdminMemo;