Add product image to powder inventory via AI lookup

When AI Lookup fetches a manufacturer product page, it now extracts the
og:image (Open Graph) meta tag before stripping HTML tags. The image URL
is returned in InventoryAiLookupResult.ImageUrl and automatically shown
as a preview on the Create/Edit form alongside the other filled fields.

The preview includes a Remove button to clear the image, and the Wrong
Match? button clears it along with the other AI-filled fields.

On the inventory Details page a product image card is rendered above the
Stock & Pricing card whenever ImageUrl is set. The field is nullable so
existing records and powders without an image are unaffected.

New field: InventoryItem.ImageUrl (nvarchar, nullable).
Migration: AddInventoryItemImageUrl.

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

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

@@ -308,6 +308,16 @@
 
             <!-- Right Column -->
             <div class="col-lg-4">
+                @if (!string.IsNullOrWhiteSpace(Model.ImageUrl))
+                {
+                    <div class="card border-0 shadow-sm mb-4">
+                        <div class="card-body p-3 text-center">
+                            <img src="@Model.ImageUrl" alt="@Model.Name"
+                                 style="max-width:100%;max-height:200px;object-fit:contain;" />
+                        </div>
+                    </div>
+                }
+
                 <!-- Stock, Pricing & Status -->
                 <div class="card border-0 shadow-sm mb-4">
                     <div class="card-header bg-white border-0 py-2 d-flex align-items-center gap-2">