INBOX-3: SMTP send + Gmail threading (POST /api/requests/{id}/send)

- IGmailMailSender (mockable) + MailKit GmailMailSender (smtp.gmail.com:587 STARTTLS, App
  Password). SendReplyService builds a threaded reply: In-Reply-To = original Message-Id,
  References = stored chain + original (deduped), Subject = 'Re: ' (no double-prefix); sends,
  then sets Status=Answered + AnsweredAt + stores sent body in DraftReply. Never auto-sends.
- POST /api/requests/{id}/send {body}: 200 updated RequestDto; 404 unknown; 503 MailNotConfigured;
  502 MailAuthFailed (UI hint: re-enter App Password). No entity change (no migration).
- Tests (fake SMTP): threading headers, Answered transition, no-double-Re, not-configured +
  auth-failure keep status; BuildMimeMessage header mapping. Live send gated on App Password.

GerbilManagerWebAPI/Inbox/GmailMailSender.cs

@@ -0,0 +1,43 @@
+using MailKit.Net.Smtp;
+using MailKit.Security;
+using MimeKit;
+
+namespace GerbilManagerWebAPI.Inbox
+{
+    /// <summary>
+    /// Live Gmail sender over SMTP (App Password auth, STARTTLS). Builds a correctly-threaded
+    /// reply so it lands in the same Gmail conversation. Gated on Julian's App Password
+    /// (tests use a fake IGmailMailSender + assert BuildMimeMessage).
+    /// </summary>
+    public sealed class GmailMailSender : IGmailMailSender
+    {
+        public async Task SendAsync(MailConnection connection, OutgoingMail mail, CancellationToken ct = default)
+        {
+            var message = BuildMimeMessage(connection.GmailAddress, mail);
+            using var client = new SmtpClient();
+            await client.ConnectAsync("smtp.gmail.com", 587, SecureSocketOptions.StartTls, ct);
+            await client.AuthenticateAsync(connection.GmailAddress, connection.AppPassword, ct);
+            await client.SendAsync(message, ct);
+            await client.DisconnectAsync(true, ct);
+        }
+
+        /// <summary>Builds the reply MimeMessage with In-Reply-To / References / Subject set for
+        /// Gmail threading. Public so the threading headers are unit-testable without SMTP.</summary>
+        public static MimeMessage BuildMimeMessage(string fromAddress, OutgoingMail mail)
+        {
+            var message = new MimeMessage();
+            message.From.Add(MailboxAddress.Parse(fromAddress));
+            message.To.Add(MailboxAddress.Parse(mail.To));
+            message.Subject = mail.Subject;
+            message.Body = new TextPart("plain") { Text = mail.Body };
+
+            if (!string.IsNullOrWhiteSpace(mail.InReplyTo))
+                message.InReplyTo = mail.InReplyTo;
+            if (!string.IsNullOrWhiteSpace(mail.References))
+                foreach (var id in mail.References.Split(' ', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+                    message.References.Add(id);
+
+            return message;
+        }
+    }
+}