Add full category path to AI price check for coating-type context

- Skip $0-priced items (placeholders/category headers) in RunAiPriceCheck
- Build full category path (e.g. "Cerakote > Firearms") via BuildCategoryPath
  so Claude receives coating-type context — Cerakote pricing differs significantly
  from standard powder coat
- Update AI system prompt to instruct Claude to use the category path when
  determining process type, equipment, cure times, and market rates

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

src/PowderCoating.Infrastructure/Services/AiCatalogPriceCheckService.cs

@@ -238,6 +238,8 @@ public class AiCatalogPriceCheckService : IAiCatalogPriceCheckService
         sb.AppendLine("- \"medium\" — reasonable assumptions were possible");
         sb.AppendLine("- \"low\" — item is too vague to estimate reliably (e.g., 'Custom Part', 'Job Special')");
         sb.AppendLine();
+        sb.AppendLine("The \"category\" field contains the full path, e.g. \"Cerakote > Firearms\" or \"Powder Coat > Wheels\". Use this to determine the coating process — Cerakote items have a very different cost profile than standard powder coat (different equipment, cure times, and market rates). Price accordingly.");
+        sb.AppendLine();
         sb.AppendLine("If the item already has an ApproximateArea or EstimatedMinutes, use those instead of guessing.");
         sb.AppendLine();
         sb.AppendLine("IMPORTANT: Keep responses concise to avoid truncation. Limit assumptions to 20 words max. Limit reasoning to 25 words max.");