feat(history,gateway): D4 会话未读/已读游标 — unreadCount + POST /sessions/{id}/read + session_read 广播

- chat_sessions 增 read_at（毫秒，与 chat_messages.created_at 同单位）；迁移 0010 并把既有会话回填为各自最新消息时间（升级当天不误报）
- ListSessions 单查询派生 unreadCount（无 N+1，复用 idx_chat_msgs_session_created 覆盖索引）
- MarkSessionRead 只前进 MAX(read_at, ?)；未知会话 ErrSessionNotFound；游标方法挂在网关消费侧窄接口（沿用 GetMessage/messageLookup 先例，避免污染 runtime 接口与其 fake）
- POST /api/v1/sessions/{id}/read：writeAuth + 400/404/501，成功广播 session_read、失败路径不广播
- Docs/api-client v0.37（§1 unreadCount / 新行 8a / §3.9 session_read / §4.2 用法）
- 含迁移回填测试、store/handler 测试与变异验证；顺带把 migrate_pinned_test 的「回退一步=0009」硬编码改为对迁移数量免疫

internal/services/gateway/routes.go

@@ -83,6 +83,10 @@ func (s *Server) registerHistoryRoutes(mux *http.ServeMux, writeAuth func(http.H
 	mux.Handle("POST /api/v1/sessions", writeAuth(http.HandlerFunc(s.handleCreateSubSession)))
 	mux.Handle("DELETE /api/v1/sessions/{id}", writeAuth(http.HandlerFunc(s.handleDeleteSession)))
 	mux.Handle("PATCH /api/v1/sessions/{id}", writeAuth(http.HandlerFunc(s.handlePatchSession)))
+	// Read cursor (D4 / Android collaboration B3): advancing it clears the session's
+	// unread badge on every end of the account, so it is an owner/admin history write
+	// like the other session mutations. It broadcasts a `session_read` frame.
+	mux.Handle("POST /api/v1/sessions/{id}/read", writeAuth(http.HandlerFunc(s.handleMarkSessionRead)))
 	// Single-message edit/delete (Android collaboration batch ③a, D3). A message is
 	// addressed by its own opaque id; both are owner/admin-gated history writes.
 	mux.Handle("PATCH /api/v1/messages/{id}", writeAuth(http.HandlerFunc(s.handlePatchMessage)))