feat: add organization and member management APIs for admin and internal use

backend/app/models/member_organization.py

@@ -0,0 +1,23 @@
+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 MemberOrganization(Base):
+    __tablename__ = "member_organizations"
+    __table_args__ = (
+        UniqueConstraint("member_id", "organization_id", name="uq_member_organizations_member_org"),
+    )
+
+    id: Mapped[str] = mapped_column(UUID(as_uuid=False), primary_key=True, default=lambda: str(uuid4()))
+    member_id: Mapped[str] = mapped_column(UUID(as_uuid=False), ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+    organization_id: Mapped[str] = mapped_column(
+        UUID(as_uuid=False), ForeignKey("organizations.id", ondelete="CASCADE"), nullable=False
+    )
+
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)