feat: AI Worker 平台 MVP v1.0.0 - 多租户/项目管理/AI Worker/任务编排/HITL审核/成本治理

backend/app/models/base.py

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+"""Base model and common mixins."""
+from datetime import datetime
+from sqlalchemy import Column, Integer, DateTime, String
+from app.database import Base
+
+
+class TimestampMixin:
+    """Adds created_at and updated_at columns."""
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+
+class TenantMixin:
+    """Adds tenant_id column for multi-tenant isolation.
+
+    Every domain model inherits this to guarantee row-level tenant isolation.
+    Queries must always filter by tenant_id.
+    """
+    tenant_id = Column(Integer, nullable=False, index=True)