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>

frontend/src/api/orders.ts

@@ -55,6 +55,7 @@ export interface OrderLine {
   render_log: RenderLog | null
   render_started_at: string | null
   render_completed_at: string | null
+  material_override: string | null
   notes: string | null
   created_at: string
   updated_at: string
@@ -66,6 +67,7 @@ export interface OrderLineCreate {
   render_position_id?: string | null
   global_render_position_id?: string | null
   gewuenschte_bildnummer?: string | null
+  material_override?: string | null
   notes?: string | null
 }
 
@@ -242,6 +244,14 @@ export async function dispatchLineRender(orderId: string, lineId: string) {
   return res.data
 }
 
+export async function patchOrderLine(orderId: string, lineId: string, data: { material_override?: string | null }) {
+  const res = await api.patch<{ updated: boolean; line_id: string }>(
+    `/orders/${orderId}/lines/${lineId}`,
+    data
+  )
+  return res.data
+}
+
 export async function cancelOrderRenders(orderId: string) {
   const res = await api.post<{ cancelled: number; order_status: string; errors: string[] | null }>(
     `/orders/${orderId}/cancel-renders`