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/migrate_pinned_test.go

@@ -40,9 +40,12 @@ func TestMigrate0009PinnedColumnIdempotent(t *testing.T) {
 		t.Fatal("chat_sessions.pinned should still exist after idempotent re-run")
 	}
 
-	// Down one step rolls back only 0009, dropping the column.
-	if err := MigrateDown(ctx, conn, 1, testLogger()); err != nil {
-		t.Fatalf("MigrateDown(1): %v", err)
+	// Roll back every migration newer than 0008 (0009 itself plus any migration added
+	// after it, currently 0010), so this test keeps validating the 0009 down no matter
+	// how many later migrations the repo has grown.
+	steps := len(upMigrationIDs(t)) - 8
+	if err := MigrateDown(ctx, conn, steps, testLogger()); err != nil {
+		t.Fatalf("MigrateDown(%d): %v", steps, err)
 	}
 	if columnExists(t, ctx, conn, "chat_sessions", "pinned") {
 		t.Fatal("chat_sessions.pinned should be dropped by the 0009 down")