feat: add action_type and focal_node_id to AI chat message API

- Add VALID_ACTION_TYPES literal and action_type/focal_node_id fields to
  AIChatMessageRequest schema
- Add tree_id field to AIChatStartRequest schema for editor-embedded sessions
- Update send_message() signature with action_type and focal_node_id params
- Update start_chat_session() signature with tree_id param
- Pass new params through endpoints to service functions
- All new params have defaults so existing behavior is unchanged

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

backend/app/api/endpoints/ai_chat.py

@@ -95,6 +95,7 @@ async def create_session(
             user_id=current_user.id,
             account_id=current_user.account_id,
             db=db,
+            tree_id=data.tree_id,
         )
     except Exception as e:
         logger.exception("AI chat session start failed: %s", e)
@@ -168,7 +169,9 @@ async def post_message(
 
     try:
         ai_content, tree_update, new_phase, metadata = await send_message(
-            session, data.content, db
+            session, data.content, db,
+            action_type=data.action_type or "open_chat",
+            focal_node_id=data.focal_node_id,
         )
     except Exception as e:
         logger.exception("AI chat message failed: %s", e)