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.Application/Interfaces/IInventoryAiLookupService.cs

@@ -25,6 +25,7 @@ public class InventoryAiLookupResult
     public decimal? UnitCostPerLb { get; set; }       // price per lb/unit if found in search results
     public string? VendorName { get; set; }           // manufacturer/vendor name for dropdown matching
     public string? SpecPageUrl { get; set; }          // URL of the product page that was fetched
+    public string? ImageUrl { get; set; }             // og:image or first product image found on the page
 
     public string? Reasoning { get; set; }       // brief explanation of what was found
 }