fix(auto_execute): ApprovalRequest 補齊 description/requested_by/required_signatures

3 validation errors 導致 auto_execute_failed
補上所有必填欄位，required_signatures=0 表示自動核准不需簽核

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

apps/api/src/services/decision_manager.py

@@ -671,12 +671,16 @@ class DecisionManager:
             from src.models.approval import ApprovalRequest, ApprovalStatus
             from src.services.approval_execution import ApprovalExecutionService
 
-            # 建立虛擬 ApprovalRequest
+            # 建立虛擬 ApprovalRequest (auto_execute — 不需人工審核)
+            _risk = token.proposal_data.get("risk_level", "low")
             approval = ApprovalRequest(
                 incident_id=incident.incident_id,
                 action=action,
+                description=token.proposal_data.get("description", action[:100]),
+                requested_by="auto_approve",
+                required_signatures=0,
                 status=ApprovalStatus.APPROVED,
-                risk_level=token.proposal_data.get("risk_level", "low"),
+                risk_level=_risk,
             )
 
             # 執行