feat: IM平台列 + deviceID标识 + 会话持久化完善

- agent-96: chat_messages 加 platform 列(迁移0004) + IM主人私聊落主会话web(带platform) + 前端按平台过滤契约 + message_persisted 带 platform(复用 BroadcastPersisted)
- agent-97: chat_messages 加 device_id 列(迁移0005) + history.Message.DeviceID + Web发消息带deviceId(localStorage getClientId) + IM persistToMain 带 deviceID + 前端气泡展示 SourceLabel + 修主会话基础2个bug(fn scanMessage语法/DDL缺列)
- design/16 §4.4(平台) + §4.5(deviceID) 已入档
- 实机: POST /chat 带 deviceId 读回 user/assistant 均带 deviceId+platform; 后端 build/vet/test 绿 + 前端 build 绿
- 红线: 身份靠 actor 元数据; 禁止 shell 读 .env(JWT冒烟由主会话跑)

internal/services/gateway/server.go

@@ -113,7 +113,7 @@ type HistoryStore interface {
 	// (with the assigned id/timestamp) so the orchestrator can broadcast it for
 	// cross-end realtime sync (design/16 §4.2).
 	AppendFull(ctx context.Context, msg history.Message) (history.Message, error)
-	List(ctx context.Context, sessionID string, before int64, limit int) ([]history.Message, error)
+	List(ctx context.Context, sessionID string, before int64, limit int, platform string) ([]history.Message, error)
 	Recent(ctx context.Context, sessionID string, n int) ([]history.Message, error)
 	DeleteSession(ctx context.Context, sessionID string) error
 	NewSubSession(ctx context.Context, ownerID string) (string, error)
@@ -459,20 +459,25 @@ func newOrchestrator(s *Server) *Orchestrator {
 		Emit: func(sessionID, typ string, data map[string]any) {
 			s.hub.broadcast(wsEvent{Type: typ, SessionID: sessionID, Data: data})
 		},
-		OnPersisted: func(msg history.Message) {
-			// Broadcast the complete message so every browser of the same account
-			// appends it without a manual refresh (design/16 §4.2). The frontend
-			// dedups by the backend-generated id, and thinking is stripped to stay
-			// consistent with the WS history replay (design/16 §6).
-			wire := toHistoryMessage(msg)
-			wire.Thinking = ""
-			s.hub.broadcast(wsEvent{Type: "message_persisted", SessionID: msg.SessionID, Data: map[string]any{"message": wire}})
-		},
-		Log:     s.log,
-		History: s.history,
+		OnPersisted: s.BroadcastPersisted,
+		Log:         s.log,
+		History:     s.history,
 	})
 }
 
+// BroadcastPersisted broadcasts a just-persisted chat message as a
+// `message_persisted` WS event so every browser of the same account appends it
+// without a manual refresh (design/16 §4.2). It is shared by the orchestrator
+// (Web turns) and the channel bridge (IM owner-private messages routed into the
+// main web session), so both paths emit the identical frame. Thinking is stripped
+// to stay consistent with the WS history replay (design/16 §6). It is a no-op-safe
+// call when the hub has broadcast wired.
+func (s *Server) BroadcastPersisted(msg history.Message) {
+	wire := toHistoryMessage(msg)
+	wire.Thinking = ""
+	s.hub.broadcast(wsEvent{Type: "message_persisted", SessionID: msg.SessionID, Data: map[string]any{"message": wire}})
+}
+
 // sourcedLog returns a logger tagged with source "gateway", substituting a
 // discard logger for nil so Server can always log safely.
 func sourcedLog(l *slog.Logger) *slog.Logger {