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/GoodsReceivedNote.cs

@@ -0,0 +1,55 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Fitness_core_BE.Models
+{
+    /// <summary>Stock coming in, optionally from a Supplier -- some GRNs don't have one (e.g.
+    /// self-produced stock, or the supplier just isn't on file). Creating one increments each
+    /// line's Product.QuantityOnHand and overwrites Product.CostPrice with that line's UnitCost
+    /// (a simple latest-cost approach, not a weighted average).</summary>
+    public class GoodsReceivedNote
+    {
+        [Key]
+        public Guid GrnId { get; set; }
+
+        public Guid FKClientId { get; set; }
+        [ForeignKey(nameof(FKClientId))]
+        public Client Client { get; set; }
+
+        public Guid? FKSupplierId { get; set; }
+        [ForeignKey(nameof(FKSupplierId))]
+        public Supplier? Supplier { get; set; }
+
+        [MaxLength(32)]
+        public string GrnNumber { get; set; } = string.Empty;
+
+        public DateTime ReceivedAt { get; set; } = DateTime.UtcNow;
+
+        [MaxLength(300)]
+        public string? Notes { get; set; }
+
+        public decimal TotalAmount { get; set; } = 0m;
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        public virtual ICollection<GoodsReceivedNoteItem> Items { get; set; } = new List<GoodsReceivedNoteItem>();
+    }
+
+    public class GoodsReceivedNoteItem
+    {
+        [Key]
+        public Guid GrnItemId { get; set; }
+
+        public Guid FKGrnId { get; set; }
+        [ForeignKey(nameof(FKGrnId))]
+        public GoodsReceivedNote? Grn { get; set; }
+
+        public Guid FKProductId { get; set; }
+        [ForeignKey(nameof(FKProductId))]
+        public Product? Product { get; set; }
+
+        public decimal Quantity { get; set; }
+        public decimal UnitCost { get; set; }
+        public decimal LineTotal { get; set; }
+    }
+}