fix(escalations): allow claimed-by user to send chat messages to escalated session

unified_chat_service.send_chat_message checked AISession.user_id == user_id,
blocking the senior who claimed an escalation from sending the AI briefing.
Now also allows AISession.escalated_to_id == user_id (the claimer).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/unified_chat_service.py

@@ -583,10 +583,14 @@ async def send_chat_message(
 
     Returns (ai_content, suggested_flows, session, fork_metadata, actions_data, questions_data).
     """
+    from sqlalchemy import or_
     result = await db.execute(
         select(AISession).where(
             AISession.id == session_id,
-            AISession.user_id == user_id,
+            or_(
+                AISession.user_id == user_id,
+                AISession.escalated_to_id == user_id,
+            ),
             AISession.session_type == "chat",
         )
     )