fix(p1-backlog): 修復「待分析」死結與 Telegram 訊息截斷

問題 1：REQUEST_REVISION → 待分析
  根因：safe_candidates=[] → selected=None → recommended_action=None
        → decision_manager action="" → TG 卡顯示「待分析」（資訊流斷裂）
  修復 coordinator_agent.py：
    無安全候選時回退至 Solver 原始最優方案
    標記「[Reviewer 未核准，僅供參考] {action}」
    SRE 永遠能看到 AI 建議，資訊流絕不中斷

問題 2：debate_summary 在 (blast_radius... 中間截斷顯示 (bl
  根因：root_cause=reasoning[:150] — 150 字元對中文 debate_summary 過短
  修復 decision_manager.py：
    root_cause 截斷 150 → 300
    suggested_action 截斷 80 → 120

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

apps/api/src/services/decision_manager.py

@@ -353,8 +353,8 @@ async def _push_decision_to_telegram(
                 approval_id=approval_id,
                 risk_level=risk_level,
                 resource_name=target[:50],
-                root_cause=reasoning[:150] if reasoning else description[:150],
-                suggested_action=action[:80] if action else "待分析",
+                root_cause=reasoning[:300] if reasoning else description[:300],
+                suggested_action=action[:120] if action else "待分析",
                 estimated_downtime="5-15 min",
                 primary_responsibility="INFRA",
                 confidence=confidence,