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/db/migrations/0010_chat_session_read_at.down.sql

@@ -0,0 +1,9 @@
+-- 0010_chat_session_read_at down: drop the read cursor added by the up migration.
+-- SQLite DROP COLUMN (3.35+) is already relied on by 0006/0007/0009, and read_at is
+-- not referenced by any index, view or constraint, so a plain drop is safe. The
+-- rolled-back state is the pre-D4 one: no unread badge data at all (the cursor is
+-- derived display state, not data a session needs to stay valid). The backfilled
+-- values cannot be restored by a re-up beyond the newest-message rule, which is
+-- exactly the upgrade default the up migration applies.
+
+ALTER TABLE chat_sessions DROP COLUMN read_at;