Add summary field and conversation summary generation logic

[johnnycrich]

What is in this PR?

Introduces automated summarization of event conversations, and saving them in database.

Changes in the codebase

• Added a summary field to the IConversation interface and Mongoose schema.
• Implemented logic in doStopConversation (lifecycle.ts) to automatically generate a summary using LLM, based on the event transcript and participant messages. The summary is stored in the conversation document.
• Added a new outgoing summary channel to the eventHistorian, potentially to be used later.

Documentation and automated testing

Did you:

• document any breaking changes in your commit messages?
• document your changes as comments in the code? Use TSDoc format where appropriate.
• update the README and docs to be clear and easy to use for end users and developers?
• add and/or update automated tests?
• update team documentation of any new or changed environment variables?

Testing this PR

• Start an event and feed enough content to generate a transcript.
• Stop the conversation and note that a summary is generated and stored in the summary field of the document in the database's conversations collection.

[jencompgeek]

Works great! This will be a very useful feature.