feat: chat pagination — infinite scroll / load older messages

- Backend: cursor-based pagination via ?before=<ISO timestamp>
- Frontend: auto-load when scrolling to top (<60px), spinner + manual 'Загрузить ещё' button
- Poll merge: preserves older history when new messages arrive from polling
- hasMoreMsgs flag: shown only when exactly 50 msgs returned (more may exist)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/chat.ts

@@ -111,7 +111,7 @@ const chatRoutes: FastifyPluginAsync = async (fastify) => {
 
   // ── GET messages ──────────────────────────────────────────────────────────
 
-  fastify.get<RoomParam & { Querystring: { limit?: string } }>(
+  fastify.get<RoomParam & { Querystring: { limit?: string; before?: string } }>(
     '/api/hotels/:slug/chat/rooms/:roomId/messages',
     { onRequest: [fastify.authenticate] },
     async (request, reply) => {
@@ -122,8 +122,13 @@ const chatRoutes: FastifyPluginAsync = async (fastify) => {
       if (!hotelId) return reply.code(404).send({ error: 'Hotel not found' })
 
       const limit = Math.min(Number(request.query.limit ?? 50), 100)
+      const before = request.query.before  // ISO timestamp cursor
       const userId = request.user.sub
 
+      const params: unknown[] = [roomId, hotelId, userId, limit]
+      const beforeClause = before ? `AND m.created_at < $5` : ''
+      if (before) params.push(before)
+
       const { rows } = await db.query(
         `SELECT m.id, m.room_id, m.sender_id, m.text, m.created_at,
                 m.is_system, m.system_name, m.attachment_url, m.edited_at, m.deleted_at,
@@ -142,17 +147,20 @@ const chatRoutes: FastifyPluginAsync = async (fastify) => {
                 ) AS reactions
            FROM chat_messages m
            LEFT JOIN users u ON u.id = m.sender_id
-          WHERE m.room_id = $1 AND m.hotel_id = $2
+          WHERE m.room_id = $1 AND m.hotel_id = $2 ${beforeClause}
           ORDER BY m.created_at DESC
           LIMIT $4`,
-        [roomId, hotelId, userId, limit],
+        params,
       )
 
-      await db.query(
-        `INSERT INTO chat_read_status (room_id, user_id, last_read) VALUES ($1, $2, NOW())
-         ON CONFLICT (room_id, user_id) DO UPDATE SET last_read = NOW()`,
-        [roomId, userId],
-      )
+      // Only mark read on initial load (no before cursor)
+      if (!before) {
+        await db.query(
+          `INSERT INTO chat_read_status (room_id, user_id, last_read) VALUES ($1, $2, NOW())
+           ON CONFLICT (room_id, user_id) DO UPDATE SET last_read = NOW()`,
+          [roomId, userId],
+        )
+      }
       return rows.reverse()
     },
   )