Surface channel delivery failures on agent replies instead of swallowing them

The previous fix made a broken SMTP/Telegram account non-fatal — the
reply gets recorded either way now — but that also made delivery
failures completely invisible: an agent sees their reply "sent"
successfully with no indication the customer never actually received it.
Added messages.deliveryFailed, set whenever the outbound send throws, and
a warning badge on the message bubble so this is now visible instead of
silent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

src/app/(admin)/tickets/[id]/ticket-thread.tsx

@@ -3,7 +3,7 @@
 import { useEffect, useRef, useState } from "react";
 import Link from "next/link";
 import { AnimatePresence, motion } from "framer-motion";
-import { ArrowLeft, Send, StickyNote, Paperclip, X, MessageSquareText } from "lucide-react";
+import { ArrowLeft, Send, StickyNote, Paperclip, X, MessageSquareText, TriangleAlert } from "lucide-react";
 import { useTicketEvents } from "@/lib/events/use-ticket-events";
 import { formatRelativeTime } from "@/lib/format";
 import { Avatar } from "@/components/avatar";
@@ -373,6 +373,15 @@ function MessageBubble({ message }: { message: MessageDTO }) {
         <p className={`text-[11px] ${isAgent ? "text-white/70" : "text-text-faint"}`}>
           {message.authorName} · {formatRelativeTime(message.createdAt)}
         </p>
+        {message.deliveryFailed && (
+          <p
+            className="mt-1 flex items-center gap-1 text-[11px] font-medium text-danger"
+            title="Сообщение сохранено, но не дошло до клиента — проверьте настройки почты/Telegram"
+          >
+            <TriangleAlert size={11} />
+            Не доставлено клиенту
+          </p>
+        )}
       </div>
     </motion.div>
   );