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/System/Errors/ErrorCodes.cs

@@ -3,12 +3,25 @@ namespace ERPCore.System.Errors;
 /// <summary>
 /// Stable domain error codes carried in the <c>code</c> extension of RFC 7807
 /// ProblemDetails responses. The authoritative catalog lives in
-/// docs/11-BACKEND-PHASE1.md; add codes here as endpoints are implemented so
+/// docs/11-BACKEND-PHASE1.md §7; add codes here as endpoints are implemented so
 /// the two stay in sync.
 /// </summary>
 public static class ErrorCodes
 {
+    // Generic (framework-level) helpers used by the exception types below.
     public const string Validation = "validation_error";
     public const string NotFound = "not_found";
     public const string Conflict = "conflict";
+
+    // Catalog (docs/11 §7) — exact strings surfaced to clients.
+    public const string SkuDuplicate = "SKU_DUPLICATE";
+    public const string MasterInUse = "MASTER_IN_USE";
+    public const string PoNotEditable = "PO_NOT_EDITABLE";
+    public const string OverReceiptTolerance = "OVER_RECEIPT_TOLERANCE";
+    public const string StockNegativeBlocked = "STOCK_NEGATIVE_BLOCKED";
+    public const string ExpiredBatchBlocked = "EXPIRED_BATCH_BLOCKED";
+    public const string OnHoldNotIssuable = "ONHOLD_NOT_ISSUABLE";
+    public const string ReasonCodeRequired = "REASON_CODE_REQUIRED";
+    public const string ConcurrencyConflict = "CONCURRENCY_CONFLICT";
+    public const string IdempotencyReplay = "IDEMPOTENCY_REPLAY";
 }