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/tests/test_auto_repair_service.py

@@ -189,7 +189,7 @@ class TestAutoRepairService:
                 affected_services=["test-service"],
             ),
             repair_steps=[],
-            success_count=5,  # < 10
+            success_count=2,  # < 3 (冷啟動門檻 2026-04-05)
             failure_count=0,
         )
         mock_playbook_service.add_playbook(playbook)