feat: 扩展 Session Manager 接口，新增 ListByUser、UpdateTitle 方法

- Manager.Create 签名新增 userID 参数
- 新增 ConversationSummary 类型和 ListByUser 分页查询
- 新增 UpdateTitle 方法
- MemoryManager 实现：ListByUser 遍历+过滤+排序，UpdateTitle，自动标题生成
- RedisManager 实现：user:{id}:sessions 索引，ListByUser 通过 SMEMBERS 查询
- AppendMessage 自动更新标题（首条 user 消息时，取前 20 字符）
- 更新 ws handler、api/session.go、orchestrator mock 的 Create 调用

backend/internal/orchestrator/pipeline_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/hhs/camtalk/internal/config"
 	"github.com/hhs/camtalk/internal/logger"
 	"github.com/hhs/camtalk/internal/models"
+	"github.com/hhs/camtalk/internal/session"
 )
 
 func init() {
@@ -63,11 +64,21 @@ type MockSessionManager struct {
 	mock.Mock
 }
 
-func (m *MockSessionManager) Create(ctx context.Context, config models.SessionConfig) (string, error) {
-	args := m.Called(ctx, config)
+func (m *MockSessionManager) Create(ctx context.Context, userID string, config models.SessionConfig) (string, error) {
+	args := m.Called(ctx, userID, config)
 	return args.String(0), args.Error(1)
 }
 
+func (m *MockSessionManager) UpdateTitle(ctx context.Context, sessionID string, title string) error {
+	args := m.Called(ctx, sessionID, title)
+	return args.Error(0)
+}
+
+func (m *MockSessionManager) ListByUser(ctx context.Context, userID string, page, size int) ([]session.ConversationSummary, int, error) {
+	args := m.Called(ctx, userID, page, size)
+	return args.Get(0).([]session.ConversationSummary), args.Int(1), args.Error(2)
+}
+
 func (m *MockSessionManager) Get(ctx context.Context, sessionID string) (*models.Session, error) {
 	args := m.Called(ctx, sessionID)
 	if args.Get(0) == nil {