fix(tree): from_dict L2 subtitle 旧格式兼容 fallback

旧格式 tree.json 无 card-level L2 subtitle 时，从已构造的 L3
children 聚合 subtitle，避免运行时加载静默丢失 L2 字幕。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/tree/index.py

@@ -20,6 +20,7 @@
 from __future__ import annotations
 
 import base64
+import dataclasses
 import json
 from dataclasses import dataclass, field
 from datetime import datetime
@@ -367,6 +368,11 @@ class L1Node:
                 state_changes=l2d["card"]["state_changes"],
                 subtitle=l2d["card"].get("subtitle", ""),
             )
+            # 旧格式兼容：L2 无 card subtitle 时从 L3 children 聚合
+            if not l2_card.subtitle:
+                l3_subs = [n.card.subtitle for n in l3_nodes if n.card.subtitle]
+                if l3_subs:
+                    l2_card = dataclasses.replace(l2_card, subtitle="\n".join(l3_subs))
             tr2 = l2d.get("time_range")
             l2_nodes.append(
                 L2Node(