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/Supplier.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Fitness_core_BE.Models
+{
+    /// <summary>Who stock is received from on a GoodsReceivedNote.</summary>
+    public class Supplier
+    {
+        [Key]
+        public Guid SupplierId { get; set; }
+
+        public Guid FKClientId { get; set; }
+        [ForeignKey(nameof(FKClientId))]
+        public Client Client { get; set; }
+
+        [MaxLength(128)]
+        public string Name { get; set; } = string.Empty;
+
+        [MaxLength(32)]
+        public string? Phone { get; set; }
+
+        [MaxLength(128)]
+        public string? Email { get; set; }
+
+        [MaxLength(300)]
+        public string? Address { get; set; }
+
+        public bool IsActive { get; set; } = true;
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    }
+}