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/runtime/history/store.go

@@ -121,10 +121,20 @@ type SessionMeta struct {
 	// A client uses it to render the conversation row without a per-session request;
 	// identity and permissions never derive from it.
 	LastMessage *MessageSummary
+	// UnreadCount is the number of messages newer than the session's read cursor
+	// (design/16 §4.2, D4), computed by ListSessions in the same query. The cursor
+	// is session-scoped and single-owner: reading on any one end marks the session
+	// read for every end of the account. Like LastMessage it is display-only —
+	// identity and permissions never derive from it.
+	UnreadCount int
 }
 
 // HistoryStore abstracts the chat-history store so callers (the gateway) and tests can
-// hold an interface rather than a concrete *Store.
+// hold an interface rather than a concrete *Store. It is deliberately kept to the calls
+// the runtime-level consumers (channelbridge, backthink) actually make: the D4 read
+// cursor (MarkSessionRead) is declared on the consumer-side interface in
+// internal/services/gateway (contracts.go) instead, so a narrower store fake elsewhere
+// does not have to implement a cursor it never advances. *Store satisfies both.
 type HistoryStore interface {
 	// Append persists msg, assigning an id and timestamp when the caller omitted them.
 	Append(ctx context.Context, msg Message) error
@@ -579,6 +589,7 @@ CREATE TABLE IF NOT EXISTS chat_sessions (
     type       TEXT NOT NULL,
     title      TEXT NOT NULL DEFAULT '',
     pinned     INTEGER NOT NULL DEFAULT 0,
+    read_at    INTEGER NOT NULL DEFAULT 0,
     created_at INTEGER NOT NULL
 )`