fix: UpdatedAt timestamp not updating when appending messages to branches

[Copilot]

The UpdatedAt field on conversation branches was not being updated when messages were appended, causing GetActiveBranch() to return stale results since it selects the branch with the most recent UpdatedAt timestamp.

Changes

• Added activeBranch.UpdatedAt = bson.NewDateTimeFromTime(time.Now()) before UpdateConversationV2() in both locations where messages are appended:
  • appendConversationMessage() - single user message append
  • CreateConversationMessageStream() - bulk message append

// Before
activeBranch.InappChatHistory = append(activeBranch.InappChatHistory, bsonMsg)
activeBranch.OpenaiChatHistoryCompletion = append(activeBranch.OpenaiChatHistoryCompletion, userOaiMsg)
if err := s.chatServiceV2.UpdateConversationV2(conversation); err != nil {
    return nil, nil, err
}

// After
activeBranch.InappChatHistory = append(activeBranch.InappChatHistory, bsonMsg)
activeBranch.OpenaiChatHistoryCompletion = append(activeBranch.OpenaiChatHistoryCompletion, userOaiMsg)
activeBranch.UpdatedAt = bson.NewDateTimeFromTime(time.Now())
if err := s.chatServiceV2.UpdateConversationV2(conversation); err != nil {
    return nil, nil, err
}

This matches the pattern already used in CreateNewBranch() which sets UpdatedAt when creating branches.

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Note

Ensures branch recency is correctly tracked so GetActiveBranch() reflects the latest message activity.

• Set activeBranch.UpdatedAt = bson.NewDateTimeFromTime(time.Now()) before UpdateConversationV2() in appendConversationMessage() and CreateConversationMessageStream()
• Added time import to support timestamp update

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