fix(11-02): correct chat API parameter name (session_id → music_session)

P0 Critical Bug Fix:
- Frontend was sending session_id but backend expects music_session
- This caused 404 errors when submitting chat messages
- Fixed in sendChatMessage function (jam-ui/src/helpers/rest.js:993)

Root cause: Parameter name mismatch between frontend and backend
- Frontend: body.session_id = sessionId
- Backend: params[:music_session]
- Result: ActiveRecord.find(nil) → 404 error

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/helpers/rest.js

@@ -990,7 +990,7 @@ export const getChatMessages = async ({ channel, sessionId, before }) => {
  */
 export const sendChatMessage = async ({ channel, sessionId, message }) => {
   const body = { channel, message };
-  if (sessionId) body.session_id = sessionId;
+  if (sessionId) body.music_session = sessionId;
 
   const baseUrl = process.env.REACT_APP_API_BASE_URL;
   const response = await fetch(`${baseUrl}/chat`, {