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.Core/Entities/InventoryItem.cs

@@ -26,6 +26,7 @@ public class InventoryItem : BaseEntity
     public string? ColorFamilies { get; set; } // Comma-separated primary color families e.g. "Green,Blue"
     public bool RequiresClearCoat { get; set; } // True if this powder requires a clear coat topcoat
     public string? SpecPageUrl { get; set; }    // Link to manufacturer's product/spec page
+    public string? ImageUrl { get; set; }       // Product image URL (sourced from og:image on AI lookup)
 
     // Sample Panel Tracking (coating category items only)
     public bool HasSamplePanel { get; set; } = false;