feat(chat): личный чат тьютор ↔ админ с вложениями

Диалоги 1:1 (один тред на тьютора), отдельный SSE-канал whatido:chat
вместо общей ленты notifications, вложения с белым списком расширений
и вставкой из буфера (Ctrl+V), очистка по chat_retention_days.
Таблицы в бэкап, chat_attachments добавлен в sweepOrphanedUploads.

public/admin.js

@@ -52,6 +52,7 @@ function buildSidebar(active) {
     { page: 'journal', label: 'Журнал', icon: 'book-open' },
     { page: 'lessons', label: 'Отчёты о занятиях', icon: 'notebook-pen' },
     { page: 'notifications', label: 'Уведомления', icon: 'bell', badge: true },
+    { page: 'chat', label: 'Чаты', icon: 'messages-square', chatBadge: true },
     { page: 'students', label: 'Ученики', icon: 'graduation-cap' },
     { page: 'groups', label: 'Группы', icon: 'users' },
     { page: 'photos', label: 'Фото', icon: 'Image' },
@@ -74,10 +75,12 @@ function buildSidebar(active) {
   const renderItem = n => {
     const isAct = n.page === active;
     const badgeHtml = n.badge ? `<span class="sidebar-badge" id="sidebarNotifyBadge" ${notifyUnread > 0 ? '' : 'hidden'}>${notifyUnread > 99 ? '99+' : notifyUnread}</span>` : '';
+    const chatBadgeHtml = n.chatBadge ? `<span class="sidebar-badge" id="sidebarChatBadge" ${chatUnread > 0 ? '' : 'hidden'}>${chatUnread > 99 ? '99+' : chatUnread}</span>` : '';
     return `<a href="${n.page}.html" data-nav="${n.page}" class="${isAct ? 'active' : ''}" title="${n.label}">
       <i data-lucide="${n.icon}"></i>
       <span class="nav-text">${n.label}</span>
       ${badgeHtml}
+      ${chatBadgeHtml}
     </a>`;
   };
   el.innerHTML = `
@@ -133,6 +136,8 @@ function buildSidebar(active) {
   loadVersion();
   loadBrand();
   initNotifications();
+  loadChatUnread();
+  connectChatStream();
 }
 
 async function loadBrand() {
@@ -218,6 +223,57 @@ function setNotifyUnread(n) {
   }
 }
 
+let chatUnread = 0;
+let chatStream = null;
+let chatReconnectTimer = null;
+
+function setChatUnread(n) {
+  chatUnread = Number(n) || 0;
+  const badge = document.getElementById('sidebarChatBadge');
+  if (badge) {
+    badge.textContent = chatUnread > 99 ? '99+' : String(chatUnread);
+    badge.hidden = chatUnread <= 0;
+  }
+}
+
+async function loadChatUnread() {
+  try {
+    const res = await fetch(`${API}/api/chat/unread`, { headers: hdr() });
+    if (!res.ok) return;
+    setChatUnread((await res.json()).unread);
+  } catch {}
+}
+
+function connectChatStream() {
+  if (chatStream) { chatStream.close(); chatStream = null; }
+  const connect = () => {
+    const t = sessionStorage.getItem('authToken');
+    if (!t) return;
+    chatStream = new EventSource(`${API}/api/chat/stream?token=${encodeURIComponent(t)}`);
+    chatStream.addEventListener('ready', ev => {
+      try { setChatUnread(JSON.parse(ev.data).unread); } catch {}
+    });
+    chatStream.addEventListener('message', ev => {
+      let p = null;
+      try { p = JSON.parse(ev.data); } catch { return; }
+      if (!p || !p.message) return;
+      const m = p.message;
+      if (m.sender_id !== currentUser?.id) {
+        setChatUnread(chatUnread + 1);
+        showToast(`Новое сообщение в чате: ${(m.sender_name || '').trim() || 'диалог'}`);
+      }
+      if (typeof onChatMessage === 'function') onChatMessage(p);
+    });
+    chatStream.onerror = () => {
+      if (chatStream) { chatStream.close(); chatStream = null; }
+      clearTimeout(chatReconnectTimer);
+      chatReconnectTimer = setTimeout(connect, 5000);
+    };
+  };
+  connect();
+  window.addEventListener('beforeunload', () => { if (chatStream) chatStream.close(); });
+}
+
 function notifyItemHtml(n, expanded) {
   const level = NOTIFY_LEVELS[n.level] ? ` level-${n.level}` : '';
   return `<a href="#" class="notify-item${n.read ? '' : ' unread'}${level}" data-notify-id="${n.id}" data-notify-link="${attrEsc(n.link || '')}">