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.

Models/Sale.cs

@@ -0,0 +1,59 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Fitness_core_BE.Models
+{
+    /// <summary>Stock going out to a customer. Creating one decrements each line's
+    /// Product.QuantityOnHand -- the whole sale is rejected up front if any line would go
+    /// negative (no backorders).</summary>
+    public class Sale
+    {
+        [Key]
+        public Guid SaleId { get; set; }
+
+        public Guid FKClientId { get; set; }
+        [ForeignKey(nameof(FKClientId))]
+        public Client Client { get; set; }
+
+        [MaxLength(32)]
+        public string SaleNumber { get; set; } = string.Empty;
+
+        public DateTime SoldAt { get; set; } = DateTime.UtcNow;
+
+        /// <summary>"Cash" | "Bank" -- same convention as Booking.PaymentMethod.</summary>
+        [MaxLength(20)]
+        public string PaymentMethod { get; set; } = "Cash";
+
+        /// <summary>Optional link to a registered member, purely for identification -- mirrors
+        /// Booking.FKCustomerId (see BookingService.ResolveOptionalCustomerLinkAsync). A walk-in
+        /// buyer leaves this null and uses CustomerName instead.</summary>
+        public Guid? FKCustomerId { get; set; }
+
+        [MaxLength(128)]
+        public string? CustomerName { get; set; }
+
+        public decimal TotalAmount { get; set; } = 0m;
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        public virtual ICollection<SaleItem> Items { get; set; } = new List<SaleItem>();
+    }
+
+    public class SaleItem
+    {
+        [Key]
+        public Guid SaleItemId { get; set; }
+
+        public Guid FKSaleId { get; set; }
+        [ForeignKey(nameof(FKSaleId))]
+        public Sale? Sale { get; set; }
+
+        public Guid FKProductId { get; set; }
+        [ForeignKey(nameof(FKProductId))]
+        public Product? Product { get; set; }
+
+        public decimal Quantity { get; set; }
+        public decimal UnitPrice { get; set; }
+        public decimal LineTotal { get; set; }
+    }
+}