Phase 3: AR/AP aging buckets, PO seeder, Bills vendor fix

- Bills.cs: replace aceHardware/fastenal lookups with grainger/harbor/localSupply
  to match Phase 1 vendor renames; update all vendor invoice number prefixes
- Bills.cs: add 3 AP aging-bucket bills (30-60, 61-90, 90+ days overdue) so all
  four AP aging buckets are populated for report demos
- Invoices.cs: add 3 more overdue invoices (31-60, 61-90, 90+ day AR buckets)
  alongside the existing 21-day overdue; total now 29 invoices
- New SeedDataService.PurchaseOrders.cs: 7 POs — 3 Received (historical), 2
  Submitted (in-flight), 2 Draft (pending approval); links to inventory items
  where available
- SeedDataService.cs: wire SeedPurchaseOrdersAsync after Vendors seeder
- Remove.cs: add PO + POItem cleanup inside Bills removal block (two-step ID
  fetch to avoid nested LINQ translation issues)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Infrastructure/Services/SeedDataService.Remove.cs

@@ -342,6 +342,34 @@ public partial class SeedDataService
                 }
             }
 
+            // --- Purchase Orders (removed alongside bills — tightly coupled in demo workflows) ---
+            if (options.Bills)
+            {
+                var poIds = await _context.Set<Core.Entities.PurchaseOrder>()
+                    .IgnoreQueryFilters()
+                    .Where(p => p.CompanyId == companyId)
+                    .Select(p => p.Id)
+                    .ToListAsync();
+
+                if (poIds.Any())
+                {
+                    var poItems = await _context.Set<Core.Entities.PurchaseOrderItem>()
+                        .IgnoreQueryFilters()
+                        .Where(i => poIds.Contains(i.PurchaseOrderId))
+                        .ToListAsync();
+                    if (poItems.Any()) _context.Set<Core.Entities.PurchaseOrderItem>().RemoveRange(poItems);
+
+                    var pos = await _context.Set<Core.Entities.PurchaseOrder>()
+                        .IgnoreQueryFilters()
+                        .Where(p => poIds.Contains(p.Id))
+                        .ToListAsync();
+                    _context.Set<Core.Entities.PurchaseOrder>().RemoveRange(pos);
+                    totalRemoved += pos.Count;
+                    details.Add($"✓ Removed {pos.Count} purchase order(s)");
+                    await _context.SaveChangesAsync();
+                }
+            }
+
             // --- Vendor Bills (all bills for the company) ---
             if (options.Bills)
             {