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.

db/init.sql

@@ -343,6 +343,42 @@ CREATE TABLE IF NOT EXISTS notification_reads (
 
 CREATE INDEX IF NOT EXISTS idx_notification_reads_user ON notification_reads(user_id);
 
+CREATE TABLE IF NOT EXISTS chat_threads (
+  id SERIAL PRIMARY KEY,
+  tutor_id INT NOT NULL UNIQUE REFERENCES users(id) ON DELETE CASCADE,
+  last_message_at TIMESTAMPTZ DEFAULT now(),
+  last_message_text VARCHAR(300),
+  tutor_unread INT NOT NULL DEFAULT 0,
+  admin_unread INT NOT NULL DEFAULT 0,
+  created_at TIMESTAMPTZ DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_chat_threads_last_message ON chat_threads(last_message_at DESC);
+
+CREATE TABLE IF NOT EXISTS chat_messages (
+  id SERIAL PRIMARY KEY,
+  thread_id INT NOT NULL REFERENCES chat_threads(id) ON DELETE CASCADE,
+  sender_id INT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  body TEXT NOT NULL DEFAULT '',
+  created_at TIMESTAMPTZ DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_chat_messages_thread ON chat_messages(thread_id, id);
+
+CREATE TABLE IF NOT EXISTS chat_attachments (
+  id SERIAL PRIMARY KEY,
+  message_id INT NOT NULL REFERENCES chat_messages(id) ON DELETE CASCADE,
+  token VARCHAR(64) UNIQUE NOT NULL,
+  path VARCHAR(255) NOT NULL,
+  name VARCHAR(255) NOT NULL,
+  size INT NOT NULL DEFAULT 0,
+  created_at TIMESTAMPTZ DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_chat_attachments_message ON chat_attachments(message_id);
+
+INSERT INTO settings (key, value) VALUES ('chat_retention_days', '365') ON CONFLICT (key) DO NOTHING;
+
 INSERT INTO settings (key, value) VALUES ('notify_enabled', 'true')
 ON CONFLICT (key) DO NOTHING;
 INSERT INTO settings (key, value) VALUES ('notify_retention_days', '30')