feat: Implement Category, Item, UOM, Vendor, and Warehouse services with CRUD operations

- Added CategoryService for managing categories with listing, tree structure, and creation functionalities.
- Introduced ItemService for item management, including listing, detail retrieval, creation, updating, and status management.
- Created UomService for handling unit of measure operations, including listing and creation.
- Developed VendorService for vendor management, supporting listing, detail retrieval, creation, updating, and status management.
- Implemented WarehouseService for warehouse and bin management, including listing warehouses, creating warehouses, and managing bins within warehouses.
- Added interfaces for each service to define the contract for service implementations.
- Generated Entity Framework Core model snapshot for database migrations.

Backend/ERPCore/Domain/Entities/Bin.cs

@@ -0,0 +1,16 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Bin / storage location within a warehouse (FR-MD-07, FR-WH-02). Stock is
+/// tracked to bin level. Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Bin
+{
+    public long BinId { get; set; }
+
+    public long WarehouseId { get; set; }
+    public Warehouse? Warehouse { get; set; }
+
+    public string Code { get; set; } = string.Empty;
+    public string? BinType { get; set; }
+}

Backend/ERPCore/Domain/Entities/Category.cs

@@ -0,0 +1,15 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Hierarchical item category (FR-MD-04). A null <see cref="ParentId"/> denotes a
+/// root category. Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Category
+{
+    public long CategoryId { get; set; }
+    public string Name { get; set; } = string.Empty;
+
+    public long? ParentId { get; set; }
+    public Category? Parent { get; set; }
+    public ICollection<Category> Children { get; set; } = new List<Category>();
+}

Backend/ERPCore/Domain/Entities/Item.cs

@@ -0,0 +1,39 @@
+using ERPCore.Domain.Enums;
+
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Item master (FR-MD-01). Mutable aggregate: carries a <see cref="RowVersion"/>
+/// concurrency token surfaced as an ETag (docs/10 Part C.10). SKU is unique.
+/// Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Item
+{
+    public long ItemId { get; set; }
+    public string Sku { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+
+    public long CategoryId { get; set; }
+    public Category? Category { get; set; }
+
+    public long BaseUomId { get; set; }
+    public Uom? BaseUom { get; set; }
+
+    public long? DefaultVendorId { get; set; }
+    public Vendor? DefaultVendor { get; set; }
+
+    public ItemType ItemType { get; set; }
+    public TrackingMode TrackingMode { get; set; }
+    public string? TaxClass { get; set; }
+    public EntityStatus Status { get; set; } = EntityStatus.Active;
+
+    public DateTime CreatedAt { get; set; }
+    public DateTime? UpdatedAt { get; set; }
+
+    /// <summary>PostgreSQL xmin-backed optimistic concurrency token (ETag source).</summary>
+    public uint RowVersion { get; set; }
+
+    public ICollection<ItemReorder> ReorderSettings { get; set; } = new List<ItemReorder>();
+    public ICollection<UomConversion> UomConversions { get; set; } = new List<UomConversion>();
+}

Backend/ERPCore/Domain/Entities/ItemReorder.cs

@@ -0,0 +1,20 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Reorder policy for an item, optionally per warehouse (FR-MD-05). Reorder alerts
+/// are computed from these versus available stock (FR-STK-10) — not stored.
+/// Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class ItemReorder
+{
+    public long ReorderId { get; set; }
+
+    public long ItemId { get; set; }
+    public Item? Item { get; set; }
+
+    public long WarehouseId { get; set; }
+    public Warehouse? Warehouse { get; set; }
+
+    public decimal ReorderPoint { get; set; }
+    public decimal ReorderQty { get; set; }
+}

Backend/ERPCore/Domain/Entities/Uom.cs

@@ -0,0 +1,11 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Unit of Measure (FR-MD-02). Referenced as an item's base UOM and as the
+/// endpoints of a <see cref="UomConversion"/>. Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Uom
+{
+    public long UomId { get; set; }
+    public string Name { get; set; } = string.Empty;
+}

Backend/ERPCore/Domain/Entities/UomConversion.cs

@@ -0,0 +1,22 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Per-item conversion factor between two UOMs (FR-MD-02/03): quantity in
+/// <see cref="FromUomId"/> × <see cref="Factor"/> = quantity in <see cref="ToUomId"/>.
+/// Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class UomConversion
+{
+    public long ConversionId { get; set; }
+
+    public long ItemId { get; set; }
+    public Item? Item { get; set; }
+
+    public long FromUomId { get; set; }
+    public Uom? FromUom { get; set; }
+
+    public long ToUomId { get; set; }
+    public Uom? ToUom { get; set; }
+
+    public decimal Factor { get; set; }
+}

Backend/ERPCore/Domain/Entities/Vendor.cs

@@ -0,0 +1,25 @@
+using ERPCore.Domain.Enums;
+
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Vendor master (FR-MD-06). Mutable aggregate with a <see cref="RowVersion"/>
+/// ETag token. Deactivated, not deleted, when referenced (FR-MD-08).
+/// Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Vendor
+{
+    public long VendorId { get; set; }
+    public string Code { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public string? Terms { get; set; }
+    public string? TaxReg { get; set; }
+    public string Currency { get; set; } = "LKR";
+    public EntityStatus Status { get; set; } = EntityStatus.Active;
+
+    public DateTime CreatedAt { get; set; }
+    public DateTime? UpdatedAt { get; set; }
+
+    /// <summary>PostgreSQL xmin-backed optimistic concurrency token (ETag source).</summary>
+    public uint RowVersion { get; set; }
+}

Backend/ERPCore/Domain/Entities/Warehouse.cs

@@ -0,0 +1,14 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Warehouse master (FR-MD-07, FR-WH-01). Owns a bin/location hierarchy.
+/// Model: docs/10-BACKEND-PHASE1.md Part C.1.
+/// </summary>
+public class Warehouse
+{
+    public long WarehouseId { get; set; }
+    public string Code { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+
+    public ICollection<Bin> Bins { get; set; } = new List<Bin>();
+}

Backend/ERPCore/Domain/Enums/EntityStatus.cs

@@ -0,0 +1,12 @@
+namespace ERPCore.Domain.Enums;
+
+/// <summary>
+/// Lifecycle status for deactivatable master data (Item, Vendor). Masters are
+/// never hard-deleted while referenced — they are set <see cref="Inactive"/>
+/// instead (FR-MD-08). Stored as a string.
+/// </summary>
+public enum EntityStatus
+{
+    Active,
+    Inactive
+}

Backend/ERPCore/Domain/Enums/ItemType.cs

@@ -0,0 +1,12 @@
+namespace ERPCore.Domain.Enums;
+
+/// <summary>
+/// Item classification (FR-MD-01). Values match the <c>itemType</c> enum in
+/// docs/11-BACKEND-PHASE1.md §8. Stored as a string in the database.
+/// </summary>
+public enum ItemType
+{
+    Stocked,
+    NonStocked,
+    Service
+}

Backend/ERPCore/Domain/Enums/TrackingMode.cs

@@ -0,0 +1,12 @@
+namespace ERPCore.Domain.Enums;
+
+/// <summary>
+/// How on-hand units of an item are individually tracked (FR-MD-01). Values match
+/// the <c>trackingMode</c> enum in docs/11-BACKEND-PHASE1.md §8. Stored as a string.
+/// </summary>
+public enum TrackingMode
+{
+    None,
+    Batch,
+    Serial
+}