feat(auto-repair): 打通自動修復閉環 (ADR-058)

問題: 告警鏈路從未呼叫 auto_repair_service，機制完全死路
修正:
1. webhooks.py: alertmanager_webhook 建立 Incident 後觸發 _try_auto_repair_background
2. playbook.py: is_high_quality 門檻降低 (冷啟動期)
   - success_count: 10 → 3
   - success_rate: 95% → 80%
3. tests: test_evaluate_not_high_quality 更新為新門檻

流程: Alertmanager → API → Incident → evaluate → P2以下+高品質Playbook → 自動執行 → Telegram通知

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

apps/api/src/models/playbook.py

@@ -237,13 +237,16 @@ class Playbook(BaseModel):
 
         條件:
         - 狀態為 APPROVED
-        - 成功率 >= 95%
-        - 成功次數 >= 10
+        - 成功率 >= 80% (冷啟動期: 原 95%，2026-04-05 ogt 降低以打破零執行惡性循環)
+        - 成功次數 >= 3  (冷啟動期: 原 10，累積足夠後再收緊)
+
+        待成熟後收緊為: success_rate >= 0.95, success_count >= 10
         """
+        # 2026-04-05 ogt: 冷啟動調整，打破 zero-execution 惡性循環
         return (
             self.status == PlaybookStatus.APPROVED
-            and self.success_rate >= 0.95
-            and self.success_count >= 10
+            and self.success_rate >= 0.80
+            and self.success_count >= 3
         )
 
     @property