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/Controllers/InventoryController.cs

@@ -240,6 +240,17 @@ public class InventoryController : Controller
             var useMetric = await _tenantContext.UseMetricSystemAsync();
             ViewBag.CoverageUnit = _measurementService.GetCoverageUnitLabel(useMetric);
 
+            // Manufacturer-level catalog status: prefer the linked catalog row, fall back to an
+            // identity match for items added before they were linked. Drives the "discontinued by
+            // manufacturer — cannot reorder" warning. This is distinct from the shop's own
+            // IsActive/DiscontinuedDate (whether the shop still stocks it).
+            var catalogItem = item.PowderCatalogItemId.HasValue
+                ? await _unitOfWork.PowderCatalog.GetByIdAsync(item.PowderCatalogItemId.Value)
+                : await FindCatalogMatchAsync(item.Manufacturer, item.ManufacturerPartNumber);
+            ViewBag.CatalogDiscontinued = catalogItem?.IsDiscontinued ?? false;
+            ViewBag.CatalogVendorName = catalogItem?.VendorName;
+            ViewBag.CatalogProductUrl = catalogItem?.ProductUrl;
+
             return View(itemDto);
         }
         catch (Exception ex)
@@ -302,6 +313,12 @@ public class InventoryController : Controller
                     item.Category = category.DisplayName;
             }
 
+            // Link to the platform catalog row when this item's identity matches one, so the detail
+            // screen can show manufacturer-level status (discontinued / cannot reorder).
+            var catalogMatch = await FindCatalogMatchAsync(item.Manufacturer, item.ManufacturerPartNumber);
+            if (catalogMatch != null)
+                item.PowderCatalogItemId = catalogMatch.Id;
+
             await _unitOfWork.InventoryItems.AddAsync(item);
             await _unitOfWork.SaveChangesAsync();
 
@@ -763,6 +780,24 @@ public class InventoryController : Controller
     /// Returns (wasInCatalog, addedToCatalog) so callers can surface UI badges.
     /// Mutates <paramref name="result"/> in place.
     /// </summary>
+    /// <summary>
+    /// Finds the platform powder catalog row matching an inventory item's identity
+    /// (Manufacturer + ManufacturerPartNumber), or null. Used to set
+    /// <see cref="InventoryItem.PowderCatalogItemId"/> and to surface manufacturer-level status
+    /// (e.g. discontinued / cannot reorder) on the detail screen.
+    /// </summary>
+    private async Task<PowderCatalogItem?> FindCatalogMatchAsync(string? manufacturer, string? sku)
+    {
+        if (string.IsNullOrWhiteSpace(manufacturer) || string.IsNullOrWhiteSpace(sku))
+            return null;
+
+        var skuLower = sku.Trim().ToLower();
+        var mfrLower = manufacturer.Trim().ToLower();
+        var hits = await _unitOfWork.PowderCatalog.FindAsync(p =>
+            p.Sku.ToLower() == skuLower && p.VendorName.ToLower().Contains(mfrLower));
+        return hits.FirstOrDefault();
+    }
+
     private async Task<(bool wasInCatalog, bool addedToCatalog)> EnrichFromCatalogAsync(
         InventoryAiLookupResult result, bool autoContribute)
     {
@@ -1257,6 +1292,7 @@ public class InventoryController : Controller
                 ColorName             = catalogItem.ColorName,
                 Manufacturer          = catalogItem.VendorName,
                 ManufacturerPartNumber= catalogItem.Sku,
+                PowderCatalogItemId   = catalogItem.Id,
                 Finish                = catalogItem.Finish,
                 ColorFamilies         = catalogItem.ColorFamilies,
                 RequiresClearCoat     = catalogItem.RequiresClearCoat ?? false,