fix(db): alert_operation_log.event_type String→PgEnum (create_type=False)

修正 DatatypeMismatchError: DB 欄位為 native enum alert_event_type，
SQLAlchemy model 誤用 String(50)，導致 alert_operation_log 寫入失敗。

使用 PgEnum(create_type=False) 讓 SQLAlchemy 映射已存在的 DB enum，
不重建型別。18 個 event_type 值與 M-003 migration 一致。

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

apps/api/src/db/models.py

@@ -25,6 +25,7 @@ from sqlalchemy import (
 from sqlalchemy import (
     Enum as SQLEnum,
 )
+from sqlalchemy.dialects.postgresql import ENUM as PgEnum
 from sqlalchemy.orm import Mapped, mapped_column
 
 from src.db.base import Base
@@ -445,7 +446,18 @@ class AlertOperationLog(Base):
     auto_repair_id: Mapped[str | None] = mapped_column(String(36), nullable=True)
 
     # 事件核心
-    event_type: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    # 2026-04-08 Claude Sonnet 4.6: Sprint 5.1 — 修正 enum 型別不符 (String→PgEnum, create_type=False)
+    event_type: Mapped[str] = mapped_column(
+        PgEnum(
+            "ALERT_RECEIVED", "TELEGRAM_SENT", "USER_ACTION", "AUTO_REPAIR_TRIGGERED",
+            "EXECUTION_STARTED", "EXECUTION_COMPLETED", "TELEGRAM_RESULT_SENT",
+            "RESOLVED", "SILENCED", "ESCALATED", "GUARDRAIL_BLOCKED",
+            "PRE_FLIGHT_PASSED", "PRE_FLIGHT_FAILED", "BACKUP_TRIGGERED",
+            "BACKUP_COMPLETED", "BACKUP_FAILED", "APPROVAL_ESCALATED", "CHANGE_APPLIED",
+            name="alert_event_type", create_type=False,
+        ),
+        nullable=False, index=True,
+    )
     actor: Mapped[str | None] = mapped_column(String(100), nullable=True, index=True)
     action_detail: Mapped[str | None] = mapped_column(String(200), nullable=True)