fix(models): approval.py 改用 timezone.utc 相容 Python 3.10

CI runner 用 Python 3.10，datetime.UTC 是 3.11 才加入。
改用 datetime.timezone.utc 全版本相容，修復 CI type-sync 全量失敗。

# 2026-04-06 ogt: root cause — CI Python 3.10 無法 import UTC

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

apps/api/src/models/approval.py

@@ -10,7 +10,7 @@ Features:
 - Pydantic 強型別驗證
 """
 
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 from enum import Enum
 from uuid import UUID, uuid4
 
@@ -106,7 +106,7 @@ class Signature(BaseModel):
     id: UUID = Field(default_factory=uuid4)
     signer_id: str = Field(..., description="簽核者 ID")
     signer_name: str = Field(..., description="簽核者名稱")
-    signed_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
+    signed_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
     comment: str | None = None
 
     # Phase 5.4.5: Telegram 審計軌跡
@@ -153,14 +153,14 @@ class ApprovalRequest(ApprovalRequestBase):
     status: ApprovalStatus = Field(default=ApprovalStatus.PENDING)
     required_signatures: int = Field(..., description="所需簽核數")
     signatures: list[Signature] = Field(default_factory=list)
-    created_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
-    updated_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
+    created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+    updated_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
     resolved_at: datetime | None = Field(default=None, description="解決時間")
     rejection_reason: str | None = Field(default=None)
     # 戰略 B: 告警風暴收斂
     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(UTC), description="最後觸發時間")
+    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")