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/Enums/AccountingEnums.cs

@@ -80,6 +80,14 @@ public enum AccountingMethod
     Accrual = 1
 }
 
+public enum VendorCreditStatus
+{
+    Open = 0,
+    PartiallyApplied = 1,
+    Applied = 2,
+    Voided = 3
+}
+
 /// <summary>Lifecycle state of a Manual Journal Entry.</summary>
 public enum JournalEntryStatus
 {