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/MailSendContracts.cs

@@ -0,0 +1,27 @@
+namespace GerbilManagerWebAPI.Inbox
+{
+    /// <summary>A reply to send, with RFC threading headers already computed.</summary>
+    public sealed record OutgoingMail(
+        string To,
+        string Subject,
+        string Body,
+        string? InReplyTo,
+        string? References);
+
+    /// <summary>Outcome of a send attempt. Error is null on success; otherwise a stable code
+    /// ("MailNotConfigured", "MailAuthFailed") the UI maps to a German hint.</summary>
+    public sealed record SendOutcome(bool Sent, string? Error)
+    {
+        public static readonly SendOutcome Ok = new(true, null);
+        public static SendOutcome Fail(string code) => new(false, code);
+    }
+
+    /// <summary>Sends mail. Abstracted so tests can assert the message/capture it without a live
+    /// SMTP server (the live MailKit/Gmail path is gated on Julian's App Password).</summary>
+    public interface IGmailMailSender
+    {
+        /// <summary>Sends the reply. Throws <see cref="MailKit.Security.AuthenticationException"/>
+        /// on bad/revoked credentials (surfaced as "MailAuthFailed").</summary>
+        Task SendAsync(MailConnection connection, OutgoingMail mail, CancellationToken ct = default);
+    }
+}