feat: material alias seeds expansion, bulk product delete, dashboard stats widgets

- Material alias seeds: 95 → 855 aliases covering German variants, DIN standards,
  Werkstoffnummern, industry terms, English equivalents, polymer abbreviations
- Batch product delete/deactivate endpoint (POST /products/batch-delete)
- Multi-select UI on Products page with floating action bar
- Dashboard: RenderThroughput + MaterialCoverage widgets
- Dashboard stats endpoint (GET /admin/dashboard-stats)

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

frontend/src/api/dashboard.ts

@@ -16,6 +16,8 @@ export type WidgetType =
   | 'TopProducts'
   | 'OrdersByUser'
   | 'RenderBackendStats'
+  | 'RenderThroughput'
+  | 'MaterialCoverage'
 
 export interface WidgetPosition {
   col: number
@@ -64,3 +66,53 @@ export async function updateTenantDefaultDashboard(
   )
   return data.widgets
 }
+
+// ── Dashboard Stats ──────────────────────────────────────────────────────────
+
+export interface RenderThroughputStats {
+  completed_today: number
+  completed_this_week: number
+  completed_this_month: number
+  failed_today: number
+  failed_this_week: number
+  failed_this_month: number
+  avg_render_time_s: number | null
+  median_render_time_s: number | null
+}
+
+export interface MaterialCoverageStats {
+  total_unique_materials: number
+  mapped_materials: number
+  unmapped_materials: number
+  coverage_pct: number
+  library_material_count: number
+  alias_count: number
+}
+
+export interface ProductStatsOverview {
+  total_products: number
+  with_step_files: number
+  without_step_files: number
+  step_coverage_pct: number
+}
+
+export interface OrderStatusBreakdown {
+  draft: number
+  submitted: number
+  processing: number
+  completed: number
+  rejected: number
+  total: number
+}
+
+export interface DashboardStats {
+  render_throughput: RenderThroughputStats
+  material_coverage: MaterialCoverageStats
+  product_stats: ProductStatsOverview
+  order_status: OrderStatusBreakdown
+}
+
+export async function getDashboardStats(): Promise<DashboardStats> {
+  const { data } = await api.get<DashboardStats>('/admin/dashboard-stats')
+  return data
+}