feat: add foreign key relations for cascade deletes to chat tables

[Kinbaum]

Summary

This PR implements the necessary changes to handle foreign key relationships between the ChatSessions table and its dependent tables ConversationMessages and WorkingMemory. This is useful if a user wants to delete a chat, we can perform a cascading deletion as defined in our schema. The FK constraint is entirely opt-in, but certain order of operations needed to change.

Code changes

• Modified saveConversation() method: Ensures the chat session row is created/updated before saving messages to prevent FK constraint violations.

Impact

• None. Users who want FK constraints can opt-in and run their own migrations
• No breaking changes to API or application logic

[vercel]

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[Kinbaum]

@pontusab Can I get your review on this PR?