feat(flywheel): 新增 GoldenCase 模型用于规则回归门禁

- 新增 golden_cases 表，承载版本化的风险规则黄金用例
  （case_key/rule_code/values_json/expected_hit/expected_severity/status）
- 注册到 db/base.py 和 models/__init__.py，进入 Base.metadata

server/src/app/models/__init__.py

@@ -14,6 +14,7 @@ from app.models.financial_record import (
     ExpenseClaim,
     ExpenseClaimItem,
 )
+from app.models.golden_case import GoldenCase
 from app.models.hermes_config import HermesTaskConfig, HermesTaskExecutionLog
 from app.models.hermes_report import HermesRiskReport
 from app.models.notification_state import NotificationState
@@ -49,6 +50,7 @@ __all__ = [
     "EmployeeChangeLog",
     "ExpenseClaim",
     "ExpenseClaimItem",
+    "GoldenCase",
     "HermesTaskConfig",
     "HermesTaskExecutionLog",
     "HermesRiskReport",