feat: per-line render overrides — override any output type setting at order time

Instead of duplicating output types for every variation (WebP vs PNG,
different resolution), keep one canonical output type and override
specific fields per order line via render_overrides JSONB.

Backend:
- render_overrides JSONB column on OrderLine (DB migration)
- Render task merges overrides with output type settings (format, width,
  height, samples, engine, denoiser, transparent_bg, cycles_device)
- POST /orders/{id}/batch-render-overrides endpoint for bulk override
- PatchLineBody accepts render_overrides for per-line patching

Frontend:
- Batch render overrides section on OrderDetail: output format dropdown
  (PNG/JPG/WebP) + resolution dropdown (512-4096)
- Clear button to remove overrides

MCP:
- create_order tool: accepts product_ids, output_type, render_overrides,
  material_override — enables "render all products as WebP" via Claude
- set_render_overrides tool: batch override on existing orders

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

frontend/src/api/orders.ts

@@ -56,6 +56,7 @@ export interface OrderLine {
   render_started_at: string | null
   render_completed_at: string | null
   material_override: string | null
+  render_overrides: Record<string, unknown> | null
   notes: string | null
   created_at: string
   updated_at: string
@@ -68,6 +69,7 @@ export interface OrderLineCreate {
   global_render_position_id?: string | null
   gewuenschte_bildnummer?: string | null
   material_override?: string | null
+  render_overrides?: Record<string, unknown> | null
   notes?: string | null
 }
 
@@ -252,7 +254,7 @@ export async function batchMaterialOverride(orderId: string, materialOverride: s
   return res.data
 }
 
-export async function patchOrderLine(orderId: string, lineId: string, data: { material_override?: string | null }) {
+export async function patchOrderLine(orderId: string, lineId: string, data: { material_override?: string | null; render_overrides?: Record<string, unknown> | null }) {
   const res = await api.patch<{ updated: boolean; line_id: string }>(
     `/orders/${orderId}/lines/${lineId}`,
     data
@@ -260,6 +262,14 @@ export async function patchOrderLine(orderId: string, lineId: string, data: { ma
   return res.data
 }
 
+export async function batchRenderOverrides(orderId: string, renderOverrides: Record<string, unknown> | null) {
+  const res = await api.post<{ updated: number; render_overrides: Record<string, unknown> | null }>(
+    `/orders/${orderId}/batch-render-overrides`,
+    { render_overrides: renderOverrides }
+  )
+  return res.data
+}
+
 export async function cancelOrderRenders(orderId: string) {
   const res = await api.post<{ cancelled: number; order_status: string; errors: string[] | null }>(
     `/orders/${orderId}/cancel-renders`