Add inventory management features including Sales, Stock Returns, and Suppliers

- Implement Sale and SaleItem models for tracking sales transactions.
- Create StockReturn model to manage stock adjustments for various reasons.
- Introduce Supplier model to represent suppliers of products.
- Develop InventoryRepository for CRUD operations on products, suppliers, sales, and stock returns.
- Implement InventoryService to handle business logic for inventory management.
- Update TenantDbContext to include new models and relationships.
- Enhance ProfitLossRepository to account for sales, purchases, and stock returns in profit and loss calculations.
- Modify Program.cs to register new inventory-related services.

Repos/ProfitLossRepository.cs

@@ -105,9 +105,68 @@ namespace Fitness_core_BE.Repos
                 })
                 .ToListAsync(ct);
 
+            // Inventory: a Sale's revenue, a GRN's cost, and a StockReturn's Amount (staff-
+            // entered -- see StockReturn's doc comment). Every return Category still posts the
+            // same way here (one Expense category, reducing profit) even though CustomerReturn
+            // is the one category that adds the stock back -- the P&L side stays uniform while
+            // the stock-direction side doesn't.
+            var productSales = await _context.Sales
+                .AsNoTracking()
+                .Where(sale =>
+                    sale.FKClientId == clientId &&
+                    sale.SoldAt >= fromUtc &&
+                    sale.SoldAt < toUtc)
+                .Select(sale => new LedgerEntryDto
+                {
+                    EntryType = "Income",
+                    Category = "Product Sales",
+                    Description = sale.SaleNumber,
+                    Amount = sale.TotalAmount,
+                    EntryDate = sale.SoldAt,
+                    IsManual = false
+                })
+                .ToListAsync(ct);
+
+            var inventoryPurchases = await _context.GoodsReceivedNotes
+                .AsNoTracking()
+                .Where(grn =>
+                    grn.FKClientId == clientId &&
+                    grn.ReceivedAt >= fromUtc &&
+                    grn.ReceivedAt < toUtc)
+                .Select(grn => new LedgerEntryDto
+                {
+                    EntryType = "Expense",
+                    Category = "Inventory Purchases",
+                    Description = grn.GrnNumber,
+                    Amount = grn.TotalAmount,
+                    EntryDate = grn.ReceivedAt,
+                    IsManual = false
+                })
+                .ToListAsync(ct);
+
+            var inventoryWriteOffs = await _context.StockReturns
+                .AsNoTracking()
+                .Where(stockReturn =>
+                    stockReturn.FKClientId == clientId &&
+                    stockReturn.ReturnedAt >= fromUtc &&
+                    stockReturn.ReturnedAt < toUtc)
+                .Select(stockReturn => new LedgerEntryDto
+                {
+                    EntryType = "Expense",
+                    Category = "Inventory Write-offs",
+                    Description = stockReturn.Category + " — " + stockReturn.Product!.Name,
+                    Amount = stockReturn.Amount,
+                    EntryDate = stockReturn.ReturnedAt,
+                    IsManual = false
+                })
+                .ToListAsync(ct);
+
             memberPayments.AddRange(platformInvoices);
             memberPayments.AddRange(staffPayments);
             memberPayments.AddRange(bookingPayments);
+            memberPayments.AddRange(productSales);
+            memberPayments.AddRange(inventoryPurchases);
+            memberPayments.AddRange(inventoryWriteOffs);
             return memberPayments;
         }