Phase D: Add Vendor Credits (AP cycle completion)

- VendorCredit, VendorCreditLineItem, VendorCreditApplication entities
- VendorCreditStatus enum (Open, PartiallyApplied, Applied, Voided)
- Migration AddVendorCredits: three new tables
- IUnitOfWork/UnitOfWork wired with all three repositories
- VendorCreditsController: Index (status tabs), Create, Details, Post, Apply, Void
- Post action: DR AP, CR each expense line (reverses original expense)
- Apply action: links credit to bill, updates Bill.AmountPaid and bill status
- Views: Index (summary cards + table), Create (dynamic line grid), Details (apply panel)
- Nav: Vendor Credits added to Finance section in _Layout

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

src/PowderCoating.Infrastructure/Data/ApplicationDbContext.cs

@@ -329,6 +329,13 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
     /// <summary>Individual debit/credit lines within a journal entry; soft-delete only (access controlled through parent JournalEntry).</summary>
     public DbSet<JournalEntryLine> JournalEntryLines { get; set; }
 
+    /// <summary>Credit notes received from vendors (returned goods, pricing disputes); tenant-filtered with soft delete.</summary>
+    public DbSet<VendorCredit> VendorCredits { get; set; }
+    /// <summary>Expense-reversal line items on a vendor credit; soft-delete only.</summary>
+    public DbSet<VendorCreditLineItem> VendorCreditLineItems { get; set; }
+    /// <summary>Application records linking a vendor credit to a specific bill; soft-delete only.</summary>
+    public DbSet<VendorCreditApplication> VendorCreditApplications { get; set; }
+
     // Job Templates
     /// <summary>Reusable job templates that pre-populate job items, coats, and prep services on job creation.</summary>
     public DbSet<JobTemplate> JobTemplates { get; set; }
@@ -624,6 +631,12 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
             !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));
         modelBuilder.Entity<JournalEntryLine>().HasQueryFilter(e => !e.IsDeleted);
 
+        // Vendor Credits: tenant-filtered; child rows soft-delete only
+        modelBuilder.Entity<VendorCredit>().HasQueryFilter(e =>
+            !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));
+        modelBuilder.Entity<VendorCreditLineItem>().HasQueryFilter(e => !e.IsDeleted);
+        modelBuilder.Entity<VendorCreditApplication>().HasQueryFilter(e => !e.IsDeleted);
+
         // Purchase Orders
         modelBuilder.Entity<PurchaseOrder>().HasQueryFilter(e =>
             !e.IsDeleted && (IsPlatformAdmin || e.CompanyId == CurrentCompanyId));
@@ -650,6 +663,20 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
             .HasForeignKey(je => je.ReversalOfId)
             .OnDelete(DeleteBehavior.Restrict);
 
+        // VendorCredit → APAccount (no cascade)
+        modelBuilder.Entity<VendorCredit>()
+            .HasOne(vc => vc.APAccount)
+            .WithMany()
+            .HasForeignKey(vc => vc.APAccountId)
+            .OnDelete(DeleteBehavior.Restrict);
+
+        // VendorCreditLineItem → Account (nullable, no cascade)
+        modelBuilder.Entity<VendorCreditLineItem>()
+            .HasOne(li => li.Account)
+            .WithMany()
+            .HasForeignKey(li => li.AccountId)
+            .OnDelete(DeleteBehavior.Restrict);
+
         // Vendor → DefaultExpenseAccount (no cascade)
         modelBuilder.Entity<Vendor>()
             .HasOne(s => s.DefaultExpenseAccount)