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/Product.cs

@@ -0,0 +1,43 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Fitness_core_BE.Models
+{
+    /// <summary>A stocked item sold at the gym (snacks, supplements, merchandise). QuantityOnHand
+    /// is a maintained running balance -- adjusted by GoodsReceivedNoteItem (+), SaleItem (-) and
+    /// StockReturn (-) -- not computed from those tables on every read.</summary>
+    public class Product
+    {
+        [Key]
+        public Guid ProductId { 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(64)]
+        public string? Sku { get; set; }
+
+        /// <summary>Relative /Uploads path to the product's photo (same convention as
+        /// ClientBusiness.BusinessLogoId) -- null when no image has been uploaded.</summary>
+        [MaxLength(300)]
+        public string? ImageId { get; set; }
+
+        [MaxLength(20)]
+        public string UnitOfMeasure { get; set; } = "pcs";
+
+        /// <summary>Set from the most recent GRN line's UnitCost -- a simple latest-cost
+        /// approach, not a weighted average.</summary>
+        public decimal CostPrice { get; set; } = 0m;
+
+        public decimal SellingPrice { get; set; } = 0m;
+
+        public decimal QuantityOnHand { get; set; } = 0m;
+
+        public bool IsActive { get; set; } = true;
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    }
+}