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/domains/materials/models.py

@@ -0,0 +1,37 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import String, DateTime, Text, ForeignKey, Integer
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+
+
+class Material(Base):
+    __tablename__ = "materials"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(200), nullable=False, unique=True)
+    description: Mapped[str] = mapped_column(Text, nullable=True)
+    source: Mapped[str] = mapped_column(String(20), nullable=False, default="manual")
+    schaeffler_code: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    created_by: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    creator: Mapped["User"] = relationship("User", foreign_keys=[created_by], lazy="select")  # type: ignore[name-defined]
+    aliases = relationship("MaterialAlias", back_populates="material", cascade="all, delete-orphan")
+
+
+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")