feat(learning): 實作 Playbook 信心度調整機制 (ADR-030)

- 新增 _promote_playbook: 高評分提升信心度 +0.1
- 新增 _demote_playbook: 低評分降低信心度 -0.15
- 新增 find_by_source_incident: 按 incident_id 查詢 Playbook
- 新增 adjust_confidence: 信心度調整 + 狀態自動轉換
- 新增 Playbook.failure_rate 屬性

自動狀態轉換:
- ai_confidence >= 0.9 + DRAFT → 自動 APPROVED
- ai_confidence < 0.3 + failure_rate > 50% → 自動 DEPRECATED

測試: 13 案例全部通過

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/models/playbook.py

@@ -208,6 +208,12 @@ class Playbook(BaseModel):
         total = self.success_count + self.failure_count
         return self.success_count / total if total > 0 else 0.0
 
+    @property
+    def failure_rate(self) -> float:
+        """失敗率 (2026-03-30 Claude Code: Learning Service 信心度調整用)"""
+        total = self.success_count + self.failure_count
+        return self.failure_count / total if total > 0 else 0.0
+
     @property
     def is_high_quality(self) -> bool:
         """