fix: 9b9ff5b 引發的 NoneAttr bug — incident_id 上移到 Base

bug: 'ApprovalRequestCreate' object has no attribute 'incident_id'
Live-fire #6 整個 webhook 500 fail。

根因: 9b9ff5b 在 approval_db 寫 request.incident_id，
但 ApprovalRequestCreate 繼承 Base 沒這 field（只在 ApprovalRequest 才有）。

修復: 把 incident_id 上移到 ApprovalRequestBase
- ApprovalRequestCreate 自動繼承 → webhook 可建帶 incident_id 的 request
- ApprovalRequest 不重複定義
- 786/786 回歸測試全過

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

apps/api/src/models/approval.py

@@ -140,6 +140,9 @@ class ApprovalRequestBase(BaseModel):
     requested_by: str = Field(..., description="請求發起者")
     expires_at: datetime | None = Field(default=None, description="到期時間")
     metadata: dict | None = Field(default=None, description="額外元資料")
+    # 2026-04-14 Claude Sonnet 4.6: 上移 incident_id 到 Base，
+    # 讓 ApprovalRequestCreate 也能攜帶（修 9b9ff5b 的 NoneAttr bug）
+    incident_id: str | None = Field(default=None, description="關聯的 Incident ID")
 
 
 class ApprovalRequestCreate(ApprovalRequestBase):
@@ -161,8 +164,7 @@ class ApprovalRequest(ApprovalRequestBase):
     fingerprint: str | None = Field(default=None, description="告警指紋 Hash")
     hit_count: int = Field(default=1, description="聚合觸發次數")
     last_seen_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc), description="最後觸發時間")
-    # 2026-04-06 ogt: 關聯 Incident — 萃取 Playbook 與 KM 寫入必須知道 incident_id
-    incident_id: str | None = Field(default=None, description="關聯的 Incident ID")
+    # 2026-04-14 Claude Sonnet 4.6: incident_id 已移至 Base（避免 ApprovalRequestCreate 缺欄位）
 
     @property
     def current_signatures(self) -> int: