Fix incoming powder inventory: defer creation to approval, deduplicate, fix category

Three bugs fixed:

1. Wrong timing — inventory items with IsIncoming=true were auto-created during
   quote save (in QuotePricingAssemblyService). Now deferred to quote approval so
   inventory only reflects powders the shop is actually going to process.

2. Duplicate records — same powder on multiple items in one quote created multiple
   inventory records. Now grouped by PowderCatalogItemId: one record per unique
   catalog powder, all matching coats linked to the same record.

3. Wrong category — category resolution used first IsCoating=true by DisplayOrder,
   which could land items in Cerakote or other unintended categories. Now prefers
   CategoryCode==POWDER explicitly, with DisplayOrder fallback.

Changes:
- QuoteItemCoat: add PowderCatalogItemId int? — persists catalog reference at quote
  save time so the approval path knows what to create
- QuotePricingAssemblyService.BuildQuoteItemCoatsAsync: store PowderCatalogItemId
  on coat instead of calling CreateIncomingInventoryItemAsync immediately
- QuotePricingAssemblyService.CreateIncomingInventoryItemAsync: signature changed
  from (coatDto, companyId) to (catalogItemId, companyId); category lookup prefers
  POWDER code; no longer clears PowderCostPerLb on the DTO
- QuotePricingAssemblyService.EnsureIncomingInventoryForApprovedQuoteAsync: new
  public method called at approval — loads pending coats, groups by catalog ID,
  creates one inventory item per group, links all coats in each group
- IQuotePricingAssemblyService: exposes EnsureIncomingInventoryForApprovedQuoteAsync
- QuotesController.ApproveQuote: calls EnsureIncomingInventory after save
- QuotesController.ChangeQuoteStatus: calls EnsureIncomingInventory on Approved
- QuoteApprovalController: injects IQuotePricingAssemblyService; calls
  EnsureIncomingInventory in ApproveInternal (customer-facing portal path)
- InventoryController.CreateIncomingFromCatalog: same category fix (prefers POWDER)
- Migration: AddPowderCatalogItemIdToCoat (nullable int on QuoteItemCoats)
- Tests: updated AddAsIncoming test to verify deferred behavior; new deduplication test

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -1222,12 +1222,16 @@ public class InventoryController : Controller
 
             var companyId = _tenantContext.GetCurrentCompanyId() ?? 0;
 
-            // Find the default coating category to assign
+            // Find the default coating category to assign.
+            // Prefer the canonical "POWDER" category (CategoryCode == "POWDER") so catalog-sourced
+            // items always land in the right bucket regardless of how many IsCoating categories exist.
             var categories = await _unitOfWork.InventoryCategoryLookups.FindAsync(c => c.CompanyId == companyId);
-            var coatingCategory = categories
-                .Where(c => c.IsActive && c.IsCoating)
-                .OrderBy(c => c.DisplayOrder)
-                .FirstOrDefault();
+            var coatingCategory = categories.FirstOrDefault(c => c.IsActive && c.IsCoating
+                                      && c.CategoryCode.Equals("POWDER", StringComparison.OrdinalIgnoreCase))
+                               ?? categories
+                                      .Where(c => c.IsActive && c.IsCoating)
+                                      .OrderBy(c => c.DisplayOrder)
+                                      .FirstOrDefault();
 
             if (coatingCategory == null)
                 return Json(new { success = false, error = "No active coating category found. Please configure inventory categories first." });