fix(telegram): root_cause 截斷上限 300→500 — 修復「質疑：無（通」幽靈重現

根因：debate_summary 結構為「診斷（≤220字）；方案；安全審查；質疑」
      診斷假設長時總長超過 300 chars → root_cause 截斷在「通」字
修復：300 → 500（Telegram 單卡 4096 限制，安全）

2026-04-17 ogt + Claude Sonnet 4.6

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

apps/api/src/services/decision_manager.py

@@ -353,7 +353,7 @@ async def _push_decision_to_telegram(
                 approval_id=approval_id,
                 risk_level=risk_level,
                 resource_name=target[:50],
-                root_cause=reasoning[:300] if reasoning else description[:300],
+                root_cause=reasoning[:500] if reasoning else description[:500],
                 suggested_action=action[:120] if action else (description[:120] if description else "待分析"),
                 estimated_downtime="5-15 min",
                 primary_responsibility="INFRA",