feat: per-order-line material override — override materials for individual renders

- Add `material_override` nullable column on OrderLine (DB migration)
- Line override takes priority over OutputType override
- PATCH /orders/{id}/lines/{id} endpoint to update material_override
- Inline dropdown on each order line in the OrderDetail page
- Amber background when override is active
- Same output type, different material per line — no need to create a new output type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/orders/models.py

@@ -150,6 +150,7 @@ class OrderLine(Base):
         ForeignKey("global_render_positions.id", ondelete="SET NULL"),
         nullable=True,
     )
+    material_override: Mapped[str | None] = mapped_column(String(200), nullable=True, default=None)
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)
     tenant_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True