refactor: rebuild backend around role-site authorization model

backend/app/models/site_role.py

@@ -0,0 +1,18 @@
+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 SiteRole(Base):
+    __tablename__ = "site_roles"
+    __table_args__ = (UniqueConstraint("site_id", "role_id", name="uq_site_roles_site_role"),)
+
+    id: Mapped[str] = mapped_column(UUID(as_uuid=False), primary_key=True, default=lambda: str(uuid4()))
+    site_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("sites.id", ondelete="CASCADE"), nullable=False)
+    role_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("roles.id", ondelete="CASCADE"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)