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/Create.cshtml

@@ -123,6 +123,17 @@
                                 </div>
                                 <span asp-validation-for="SpecPageUrl" class="text-danger"></span>
                             </div>
+                            <input asp-for="ImageUrl" type="hidden" id="field-imageurl" />
+                            <div class="col-12" id="wrap-imagepreview" style="display:@(Model.ImageUrl != null ? "" : "none");">
+                                <label class="form-label text-muted small">Product Image (from AI Lookup)</label>
+                                <div class="d-flex align-items-start gap-3">
+                                    <img id="field-imagepreview-img" src="@Model.ImageUrl" alt="Product image"
+                                         style="max-height:120px;max-width:160px;object-fit:contain;border:1px solid #dee2e6;border-radius:6px;background:#f8f9fa;padding:4px;" />
+                                    <button type="button" class="btn btn-sm btn-outline-danger" id="btn-clear-image" title="Remove image">
+                                        <i class="bi bi-x me-1"></i>Remove
+                                    </button>
+                                </div>
+                            </div>
                             <div class="col-md-6" id="wrap-coverage">
                                 <div class="d-flex align-items-center gap-1 mb-1">
                                     <label asp-for="CoverageSqFtPerLb" class="form-label mb-0">Coverage (@ViewBag.CoverageUnit)</label>