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.Infrastructure/Data/ApplicationDbContext.cs

@@ -809,6 +809,15 @@ modelBuilder.Entity<ReworkRecord>().HasQueryFilter(e =>
             .HasForeignKey(s => s.DefaultExpenseAccountId)
             .OnDelete(DeleteBehavior.SetNull);
 
+        // Vendor ↔ InventoryCategoryLookup (many-to-many supply categories)
+        modelBuilder.Entity<Vendor>()
+            .HasMany(v => v.Categories)
+            .WithMany(c => c.Vendors)
+            .UsingEntity<Dictionary<string, object>>(
+                "VendorInventoryCategories",
+                j => j.HasOne<InventoryCategoryLookup>().WithMany().HasForeignKey("InventoryCategoryLookupId"),
+                j => j.HasOne<Vendor>().WithMany().HasForeignKey("VendorId"));
+
         // Bill → APAccount (no cascade to avoid cycles)
         modelBuilder.Entity<Bill>()
             .HasOne(b => b.APAccount)