Link inventory to powder catalog and flag discontinued items

Phase 5 (part): the inventory tie-in.

- Set InventoryItem.PowderCatalogItemId on the catalog-sourced create paths:
  directly in CreateIncomingFromCatalog, and via a new FindCatalogMatchAsync
  (Manufacturer + ManufacturerPartNumber) helper in Create.
- Inventory Details loads the linked catalog row (falling back to an identity
  match for items created before linking) and shows a "Discontinued by
  manufacturer — cannot reorder" badge + banner when it's discontinued.
  Deliberately distinct from the shop's own Active/Inactive status: existing
  stock can still be used and quoted, it just can't be reordered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Inventory/Details.cshtml

@@ -69,6 +69,12 @@
                     {
                         <span class="badge bg-danger"><i class="bi bi-x-circle me-1"></i>Inactive</span>
                     }
+                    @if ((bool?)ViewBag.CatalogDiscontinued == true)
+                    {
+                        <span class="badge bg-warning text-dark" title="Discontinued by the manufacturer — cannot reorder">
+                            <i class="bi bi-slash-circle me-1"></i>Discontinued by manufacturer
+                        </span>
+                    }
                 </div>
             </div>
             <div class="d-flex gap-2">
@@ -103,6 +109,20 @@
                 <div><strong>Status:</strong> This item is inactive</div>
             </div>
         }
+        @if ((bool?)ViewBag.CatalogDiscontinued == true)
+        {
+            <div class="alert alert-warning alert-permanent d-flex align-items-center mb-3">
+                <i class="bi bi-slash-circle me-2"></i>
+                <div>
+                    <strong>Discontinued by @(ViewBag.CatalogVendorName ?? "manufacturer"):</strong>
+                    this powder has been discontinued and cannot be reordered. Existing stock can still be used and quoted.
+                    @if (!string.IsNullOrEmpty(ViewBag.CatalogProductUrl as string))
+                    {
+                        <a href="@ViewBag.CatalogProductUrl" target="_blank" rel="noopener" class="alert-link ms-1">View product page</a>
+                    }
+                </div>
+            </div>
+        }
 
         <div class="row g-4">
             <!-- Left Column -->