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/Repositories/UnitOfWork.cs

@@ -146,6 +146,11 @@ public class UnitOfWork : IUnitOfWork
     private IRepository<JournalEntry>? _journalEntries;
     private IRepository<JournalEntryLine>? _journalEntryLines;
 
+    // Vendor Credits
+    private IRepository<VendorCredit>? _vendorCredits;
+    private IRepository<VendorCreditLineItem>? _vendorCreditLineItems;
+    private IRepository<VendorCreditApplication>? _vendorCreditApplications;
+
     /// <summary>
     /// Initialises the unit of work with the scoped <paramref name="context"/>.
     /// The context is shared across all repositories created by this instance so that
@@ -526,6 +531,19 @@ public class UnitOfWork : IUnitOfWork
     public IRepository<JournalEntryLine> JournalEntryLines =>
         _journalEntryLines ??= new Repository<JournalEntryLine>(_context);
 
+    // Vendor Credits
+    /// <summary>Repository for <see cref="VendorCredit"/> credit notes received from vendors; tenant-filtered with soft delete.</summary>
+    public IRepository<VendorCredit> VendorCredits =>
+        _vendorCredits ??= new Repository<VendorCredit>(_context);
+
+    /// <summary>Repository for <see cref="VendorCreditLineItem"/> expense-reversal lines on a vendor credit.</summary>
+    public IRepository<VendorCreditLineItem> VendorCreditLineItems =>
+        _vendorCreditLineItems ??= new Repository<VendorCreditLineItem>(_context);
+
+    /// <summary>Repository for <see cref="VendorCreditApplication"/> records linking a vendor credit to a specific bill.</summary>
+    public IRepository<VendorCreditApplication> VendorCreditApplications =>
+        _vendorCreditApplications ??= new Repository<VendorCreditApplication>(_context);
+
     /// <summary>
     /// Flushes all pending changes in the EF Core change tracker to the database.
     /// Returns the number of state entries written.