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);

src/PowderCoating.Web/Controllers/VendorsController.cs

@@ -181,6 +181,7 @@ public class VendorsController : Controller
     public async Task<IActionResult> Create(bool inline = false)
     {
         await PopulateExpenseAccountsAsync();
+        await PopulateVendorCategoriesAsync();
         if (inline)
             return PartialView(new CreateVendorDto());
         return View(new CreateVendorDto());
@@ -207,6 +208,7 @@ public class VendorsController : Controller
                 return Json(new { success = false, errors });
             }
             await PopulateExpenseAccountsAsync();
+            await PopulateVendorCategoriesAsync(dto.CategoryIds);
             return View(dto);
         }
 
@@ -216,6 +218,12 @@ public class VendorsController : Controller
             var vendor = _mapper.Map<Vendor>(dto);
             vendor.CompanyId = currentUser!.CompanyId;
 
+            if (dto.CategoryIds.Any())
+            {
+                var cats = await _unitOfWork.InventoryCategoryLookups.FindAsync(c => dto.CategoryIds.Contains(c.Id));
+                vendor.Categories = cats.ToList();
+            }
+
             await _unitOfWork.Vendors.AddAsync(vendor);
             await _unitOfWork.CompleteAsync();
 
@@ -247,14 +255,16 @@ public class VendorsController : Controller
 
         try
         {
-            var vendor = await _unitOfWork.Vendors.GetByIdAsync(id.Value);
+            var vendor = await _unitOfWork.Vendors.GetByIdAsync(id.Value, false, v => v.Categories);
             if (vendor == null)
             {
                 return NotFound();
             }
 
             var dto = _mapper.Map<UpdateVendorDto>(vendor);
+            dto.CategoryIds = vendor.Categories.Select(c => c.Id).ToList();
             await PopulateExpenseAccountsAsync();
+            await PopulateVendorCategoriesAsync(dto.CategoryIds);
             return View(dto);
         }
         catch (Exception ex)
@@ -282,18 +292,27 @@ public class VendorsController : Controller
         if (!ModelState.IsValid)
         {
             await PopulateExpenseAccountsAsync();
+            await PopulateVendorCategoriesAsync(dto.CategoryIds);
             return View(dto);
         }
 
         try
         {
-            var vendor = await _unitOfWork.Vendors.GetByIdAsync(id);
+            var vendor = await _unitOfWork.Vendors.GetByIdAsync(id, false, v => v.Categories);
             if (vendor == null)
             {
                 return NotFound();
             }
 
             _mapper.Map(dto, vendor);
+
+            vendor.Categories.Clear();
+            if (dto.CategoryIds.Any())
+            {
+                var cats = await _unitOfWork.InventoryCategoryLookups.FindAsync(c => dto.CategoryIds.Contains(c.Id));
+                foreach (var cat in cats) vendor.Categories.Add(cat);
+            }
+
             await _unitOfWork.Vendors.UpdateAsync(vendor);
             await _unitOfWork.CompleteAsync();
 
@@ -413,6 +432,20 @@ public class VendorsController : Controller
     /// purchases) in addition to regular operating expenses. A "— None —" placeholder is prepended so
     /// the field is optional — not every vendor needs a default account pre-set.
     /// </summary>
+    /// <summary>
+    /// Populates ViewBag.VendorCategories with active inventory categories for the checkbox list,
+    /// and ViewBag.SelectedCategoryIds with the IDs already assigned to the vendor being edited.
+    /// </summary>
+    private async Task PopulateVendorCategoriesAsync(IEnumerable<int>? selectedIds = null)
+    {
+        var companyId = (await _userManager.GetUserAsync(User))!.CompanyId;
+        var cats = (await _unitOfWork.InventoryCategoryLookups.FindAsync(c => c.CompanyId == companyId && c.IsActive))
+            .OrderBy(c => c.DisplayOrder)
+            .ToList();
+        ViewBag.VendorCategories = cats;
+        ViewBag.SelectedCategoryIds = (selectedIds ?? Enumerable.Empty<int>()).ToHashSet();
+    }
+
     private async Task PopulateExpenseAccountsAsync()
     {
         var accounts = (await _unitOfWork.Accounts.FindAsync(