feat: split initial social schema migration chain

replace monolithic migration with ordered scripts, include profiles/sessions in migration, and verify full downgrade/upgrade cycle for clean Supabase bootstrap

backend/src/models/todo_sources.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+import uuid
+
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.db.base import Base, TimestampMixin
+
+
+class TodoSource(TimestampMixin, Base):
+    __tablename__: str = "todo_sources"
+    __table_args__ = {"extend_existing": True}
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    todo_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        nullable=False,
+    )
+    schedule_item_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        nullable=False,
+    )