Warn on label scan when product already exists in tenant inventory

After resolving manufacturer + SKU from the scan, ScanLabel now queries the
tenant's InventoryItems: first by ManufacturerPartNumber exact match (most
precise), then by ColorName + Manufacturer fuzzy match as fallback.

If a match is found, the response includes existingInventoryId and
existingInventoryName. The JS fillFromScan() shows a warning banner with a
direct link to the existing item instead of the normal success message. Form
fields are still pre-filled so the user can proceed to add a new entry (e.g.
a different lot or bag size) if that was the intent.

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -834,6 +834,36 @@ public class InventoryController : Controller
             }
         }
 
+        // Check if this product already exists in the tenant's inventory.
+        // Match by ManufacturerPartNumber first (most precise); fall back to color name + manufacturer.
+        // Returns the first active match so the UI can warn the user before they create a duplicate.
+        int?    existingInventoryId   = null;
+        string? existingInventoryName = null;
+
+        if (!string.IsNullOrEmpty(sku))
+        {
+            var skuLower = sku.ToLower();
+            var byPart = await _unitOfWork.InventoryItems.FindAsync(i =>
+                i.ManufacturerPartNumber != null &&
+                i.ManufacturerPartNumber.ToLower() == skuLower);
+            var hit = byPart.FirstOrDefault();
+            if (hit != null) { existingInventoryId = hit.Id; existingInventoryName = hit.Name; }
+        }
+
+        if (existingInventoryId == null && !string.IsNullOrEmpty(colorName))
+        {
+            var nameLower = colorName.ToLower();
+            var mfrLower  = manufacturer?.ToLower() ?? "";
+            var byName = await _unitOfWork.InventoryItems.FindAsync(i =>
+                (i.ColorName != null && i.ColorName.ToLower() == nameLower) ||
+                i.Name.ToLower() == nameLower);
+            var hit = byName.FirstOrDefault(i =>
+                string.IsNullOrEmpty(mfrLower) ||
+                (i.Manufacturer ?? "").ToLower().Contains(mfrLower) ||
+                mfrLower.Contains((i.Manufacturer ?? "").ToLower().Trim()));
+            if (hit != null) { existingInventoryId = hit.Id; existingInventoryName = hit.Name; }
+        }
+
         return Json(new
         {
             success               = true,
@@ -856,6 +886,8 @@ public class InventoryController : Controller
             vendorName            = manufacturer,
             wasInCatalog          = wasInCatalog,
             addedToCatalog        = addedToCatalog,
+            existingInventoryId   = existingInventoryId,
+            existingInventoryName = existingInventoryName,
             reasoning             = aiResult.Reasoning,
         });
     }

src/PowderCoating.Web/wwwroot/js/inventory-label-scan.js

@@ -404,7 +404,16 @@
                 ? ' <span class="badge bg-success ms-1">Added to platform catalog</span>'
                 : '';
 
-        if (filled.length > 0) {
+        if (data.existingInventoryId) {
+            const itemName = data.existingInventoryName || data.colorName || 'This product';
+            const filledNote = filled.length > 0 ? ` Fields pre-filled from scan.` : '';
+            showFormStatus('warning',
+                `<i class="bi bi-exclamation-triangle-fill me-1"></i>` +
+                `<strong>${itemName}</strong> is already in your inventory. ` +
+                `<a href="/Inventory/Details/${data.existingInventoryId}" class="alert-link fw-semibold">View existing item</a>` +
+                ` &mdash; or continue below to add a new entry (e.g. a new lot or bag size).${filledNote}${catalogNote}`
+            );
+        } else if (filled.length > 0) {
             showFormStatus('success', `Filled from label scan: ${filled.join(', ')}.${catalogNote}`);
         } else {
             showFormStatus('warning', `Label scanned but no empty fields to fill.${catalogNote}`);