feat(FN-1975): stream chat session updates through SSE

- Emit chat:session:updated from ChatStore when message deletion mutates a session and cover deleteMessage false returns
- Pass ChatStore into SSE setup and forward chat session update events to connected clients
- Update useChat to consume SSE updates in real time with safer EnrichedChatSession typing
- Add core and dashboard tests for chat-store emissions, SSE forwarding, route wiring, and hook behavior

packages/core/src/index.ts

@@ -588,6 +588,7 @@ export type {
   ChatMessageRole,
   ChatSession,
   ChatSessionSummary,
+  EnrichedChatSession,
   ChatMention,
   ChatMessage,
   ChatMessageCreateInput,