Add vendor supply categories with inventory auto-filter

Vendors can now be tagged with one or more inventory categories (Powder,
Chemical, etc.) via checkboxes on the Create/Edit form. The inventory
Create/Edit vendor dropdown automatically filters to matching vendors when
a category is selected; falls back to all vendors if none are tagged.
Includes migration AddVendorCategories (VendorInventoryCategories join table).

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -1495,8 +1495,20 @@ public class InventoryController : Controller
         var companyId = _tenantContext.GetCurrentCompanyId() ?? 0;
         ViewBag.AiInventoryAssistEnabled = await _subscriptionService.IsAiInventoryAssistEnabledAsync(companyId);
 
-        var vendors = await _unitOfWork.Vendors.FindAsync(v => v.CompanyId == companyId);
-        ViewBag.Vendors = new SelectList(vendors.Where(s => s.IsActive).OrderBy(s => s.CompanyName), "Id", "CompanyName");
+        var vendors = (await _unitOfWork.Vendors.FindAsync(v => v.IsActive, false, v => v.Categories))
+            .OrderBy(v => v.CompanyName).ToList();
+        ViewBag.Vendors = new SelectList(vendors, "Id", "CompanyName");
+
+        // Build {categoryId: [vendorId, ...]} so the inventory form can filter vendors by category
+        var categoryVendorMap = new Dictionary<string, List<int>>();
+        foreach (var v in vendors)
+            foreach (var cat in v.Categories)
+            {
+                var key = cat.Id.ToString();
+                if (!categoryVendorMap.ContainsKey(key)) categoryVendorMap[key] = new List<int>();
+                categoryVendorMap[key].Add(v.Id);
+            }
+        ViewBag.CategoryVendorMapJson = System.Text.Json.JsonSerializer.Serialize(categoryVendorMap);
 
         // Load categories from lookup table
         var allCategories = await _unitOfWork.InventoryCategoryLookups.FindAsync(c => c.CompanyId == companyId);