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.Core/Entities/Accounting.cs

@@ -200,3 +200,59 @@ public class JournalEntryLine : BaseEntity
     public virtual JournalEntry JournalEntry { get; set; } = null!;
     public virtual Account Account { get; set; } = null!;
 }
+
+/// <summary>
+/// A credit note received from a vendor (returned goods, pricing dispute, short-ship).
+/// Reduces Accounts Payable and reverses the original expense/COGS when posted.
+/// Numbering: VC-YYMM-####
+/// </summary>
+public class VendorCredit : BaseEntity
+{
+    public string CreditNumber { get; set; } = string.Empty;
+    public int VendorId { get; set; }
+    /// <summary>AP account this credit reduces (default: Accounts Payable 2000).</summary>
+    public int APAccountId { get; set; }
+    public DateTime CreditDate { get; set; } = DateTime.UtcNow;
+    public VendorCreditStatus Status { get; set; } = VendorCreditStatus.Open;
+    public decimal Total { get; set; }
+    public decimal RemainingAmount { get; set; }
+    public string? Memo { get; set; }
+
+    // Navigation
+    public virtual Vendor Vendor { get; set; } = null!;
+    public virtual Account APAccount { get; set; } = null!;
+    public virtual ICollection<VendorCreditLineItem> LineItems { get; set; } = new List<VendorCreditLineItem>();
+    public virtual ICollection<VendorCreditApplication> Applications { get; set; } = new List<VendorCreditApplication>();
+}
+
+/// <summary>
+/// A single line on a vendor credit, each reversing a specific expense/COGS account.
+/// </summary>
+public class VendorCreditLineItem : BaseEntity
+{
+    public int VendorCreditId { get; set; }
+    /// <summary>Expense/COGS account being reversed by this line.</summary>
+    public int? AccountId { get; set; }
+    public string Description { get; set; } = string.Empty;
+    public decimal Amount { get; set; }
+
+    // Navigation
+    public virtual VendorCredit VendorCredit { get; set; } = null!;
+    public virtual Account? Account { get; set; }
+}
+
+/// <summary>
+/// Records the application of a vendor credit against a specific vendor bill.
+/// No additional GL posting is needed — AP was already adjusted when the credit was posted.
+/// </summary>
+public class VendorCreditApplication : BaseEntity
+{
+    public int VendorCreditId { get; set; }
+    public int BillId { get; set; }
+    public decimal Amount { get; set; }
+    public DateTime AppliedDate { get; set; } = DateTime.UtcNow;
+
+    // Navigation
+    public virtual VendorCredit VendorCredit { get; set; } = null!;
+    public virtual Bill Bill { get; set; } = null!;
+}