refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/material_alias.py

@@ -1,19 +1,3 @@
-import uuid
-from datetime import datetime
-from sqlalchemy import String, DateTime, ForeignKey
-from sqlalchemy.orm import Mapped, mapped_column, relationship
-from sqlalchemy.dialects.postgresql import UUID
-from app.database import Base
-
-
-class MaterialAlias(Base):
-    __tablename__ = "material_aliases"
-
-    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-    material_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("materials.id", ondelete="CASCADE"), nullable=False
-    )
-    alias: Mapped[str] = mapped_column(String(300), nullable=False)
-    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
-
-    material = relationship("Material", back_populates="aliases")
+# Compat shim — use app.domains.materials.models instead
+from app.domains.materials.models import MaterialAlias
+__all__ = ["MaterialAlias"]