Implement store credit tracking and enforcement

Introduced StoreCreditMovement entity and migration to track store credit usage and settlements. Updated payment flow to enforce credit limits and record credit movements. Modified outstanding balance calculation to use credit movements. Enhanced frontend to display real-time credit info and prevent over-credit payments. Updated API types, seed data, and DI registrations accordingly.

Distribution/Features/StoreCredit/IStoreCreditMovementRepository.cs

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+using Distribution.Models;
+
+namespace Distribution.Features.StoreCredit;
+
+public interface IStoreCreditMovementRepository
+{
+    Task<decimal> GetOutstandingForStoreAsync(Guid storeId);
+    Task<List<StoreCreditMovement>> GetByStoreAsync(Guid storeId, int page, int pageSize);
+    Task<int> GetCountByStoreAsync(Guid storeId);
+    Task AddAsync(StoreCreditMovement movement);
+}

Distribution/Features/StoreCredit/StoreCreditMovementRepository.cs

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+using Distribution.Data;
+using Distribution.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace Distribution.Features.StoreCredit;
+
+public class StoreCreditMovementRepository(AppDBContext context) : IStoreCreditMovementRepository
+{
+    public async Task<decimal> GetOutstandingForStoreAsync(Guid storeId)
+    {
+        var usage = await context.StoreCreditMovements
+            .Where(m => m.IsActive && m.FK_StoreId == storeId && m.Direction == StoreCreditMovementDirection.Usage)
+            .SumAsync(m => (decimal?)m.Amount) ?? 0m;
+
+        var settlements = await context.StoreCreditMovements
+            .Where(m => m.IsActive && m.FK_StoreId == storeId && m.Direction == StoreCreditMovementDirection.Settlement)
+            .SumAsync(m => (decimal?)m.Amount) ?? 0m;
+
+        return usage - settlements;
+    }
+
+    public async Task<List<StoreCreditMovement>> GetByStoreAsync(Guid storeId, int page, int pageSize) =>
+        await context.StoreCreditMovements
+            .Where(m => m.IsActive && m.FK_StoreId == storeId)
+            .OrderByDescending(m => m.OccurredAt)
+            .Skip((page - 1) * pageSize)
+            .Take(pageSize)
+            .ToListAsync();
+
+    public async Task<int> GetCountByStoreAsync(Guid storeId) =>
+        await context.StoreCreditMovements
+            .CountAsync(m => m.IsActive && m.FK_StoreId == storeId);
+
+    public async Task AddAsync(StoreCreditMovement movement) =>
+        await context.StoreCreditMovements.AddAsync(movement);
+}