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/schemas.py

@@ -66,6 +66,7 @@ class OrderLineCreate(BaseModel):
     render_position_id: uuid.UUID | None = None
     global_render_position_id: uuid.UUID | None = None
     gewuenschte_bildnummer: str | None = None
+    material_override: str | None = None
     notes: str | None = None
 
 
@@ -91,6 +92,7 @@ class OrderLineOut(BaseModel):
     render_log: dict | None = None
     render_started_at: datetime | None = None
     render_completed_at: datetime | None = None
+    material_override: str | None = None
     notes: str | None
     created_at: datetime
     updated_at: datetime