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.

Interfaces/IInventoryRepository.cs

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+using Fitness_core_BE.Models;
+
+namespace Fitness_core_BE.Interfaces
+{
+    public interface IInventoryRepository
+    {
+        // Products
+        Task<List<Product>> GetProducts(Guid clientId, bool includeInactive = false);
+        Task<Product?> GetProductById(Guid productId);
+        Task<Product> CreateProduct(Product product);
+        Task<Product> UpdateProduct(Product product);
+        /// <summary>Next sequential number for this client, e.g. 3 -&gt; "PRD-0004" -- Sku is
+        /// system-generated, never user-entered.</summary>
+        Task<int> GetNextProductSequence(Guid clientId);
+
+        // Product detail page: stock-movement history (GRN-in, Sale-out, Return-out).
+        Task<List<GoodsReceivedNoteItem>> GetGrnItemsForProduct(Guid productId);
+        Task<List<SaleItem>> GetSaleItemsForProduct(Guid productId);
+        Task<List<StockReturn>> GetStockReturnsForProduct(Guid productId);
+
+        // Suppliers
+        Task<List<Supplier>> GetSuppliers(Guid clientId, bool includeInactive = false);
+        Task<Supplier?> GetSupplierById(Guid supplierId);
+        Task<Supplier> CreateSupplier(Supplier supplier);
+
+        // GRN
+        Task<GoodsReceivedNote> CreateGrn(GoodsReceivedNote grn);
+        Task<List<GoodsReceivedNote>> GetGrns(Guid clientId);
+        Task<GoodsReceivedNote?> GetGrnById(Guid grnId);
+        /// <summary>Next sequential number for this client, e.g. 3 -&gt; "GRN-0004".</summary>
+        Task<int> GetNextGrnSequence(Guid clientId);
+
+        // Sales
+        Task<Sale> CreateSale(Sale sale);
+        Task<List<Sale>> GetSales(Guid clientId);
+        Task<Sale?> GetSaleById(Guid saleId);
+        Task<int> GetNextSaleSequence(Guid clientId);
+
+        // Returns
+        Task<StockReturn> CreateStockReturn(StockReturn stockReturn);
+        Task<List<StockReturn>> GetStockReturns(Guid clientId);
+        Task<StockReturn?> GetStockReturnById(Guid stockReturnId);
+    }
+}

Interfaces/Services/IInventoryService.cs

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+namespace Fitness_core_BE.Interfaces.Services
+{
+    public interface IInventoryService
+    {
+        Task<object> GetProducts(object payload, HttpContext httpContext);
+        Task<object> GetProductById(object payload, HttpContext httpContext);
+        Task<object> CreateProduct(object payload, HttpContext httpContext);
+        Task<object> UpdateProduct(object payload, HttpContext httpContext);
+        Task<object> UploadProductImage(Guid productId, IFormFile image, HttpContext httpContext);
+
+        Task<object> GetSuppliers(object payload, HttpContext httpContext);
+        Task<object> CreateSupplier(object payload, HttpContext httpContext);
+
+        Task<object> CreateGrn(object payload, HttpContext httpContext);
+        Task<object> GetGrns(object payload, HttpContext httpContext);
+        Task<object> GetGrnById(object payload, HttpContext httpContext);
+
+        Task<object> CreateSale(object payload, HttpContext httpContext);
+        Task<object> GetSales(object payload, HttpContext httpContext);
+        Task<object> GetSaleById(object payload, HttpContext httpContext);
+
+        Task<object> CreateStockReturn(object payload, HttpContext httpContext);
+        Task<object> GetStockReturns(object payload, HttpContext httpContext);
+        Task<object> GetStockReturnById(object payload, HttpContext httpContext);
+    }
+}