refactor: rebuild backend around role-site authorization model

app/models/user_site.py

@@ -0,0 +1,21 @@
+from datetime import datetime
+from uuid import uuid4
+
+from sqlalchemy import DateTime, ForeignKey, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+
+
+class UserSite(Base):
+    __tablename__ = "user_sites"
+    __table_args__ = (UniqueConstraint("user_id", "site_id", name="uq_user_sites_user_site"),)
+
+    id: Mapped[str] = mapped_column(UUID(as_uuid=False), primary_key=True, default=lambda: str(uuid4()))
+    user_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    site_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("sites.id", ondelete="CASCADE"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False
+    )