feat: 初始提交

backend/app/models/usage_metric.py

@@ -0,0 +1,13 @@
+from sqlalchemy import Integer, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from ..db.base import Base
+
+
+class UsageMetric(Base):
+    """通用计数器表，目前用于记录 API 请求次数等统计数据。"""
+
+    __tablename__ = "usage_metrics"
+
+    key: Mapped[str] = mapped_column(String(64), primary_key=True)
+    value: Mapped[int] = mapped_column(Integer, nullable=False, default=0)