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/Common/Http/ETag.cs

@@ -0,0 +1,37 @@
+namespace ERPCore.Common.Http;
+
+/// <summary>
+/// Encodes the PostgreSQL xmin concurrency token (a <see cref="uint"/>) as an
+/// opaque, quoted HTTP ETag and parses <c>If-Match</c> values back. Round-trips
+/// via base64 so the value is stable and content-type agnostic
+/// (docs/11-BACKEND-PHASE1.md §1.6).
+/// </summary>
+public static class ETag
+{
+    /// <summary>Quoted ETag string for a row-version token, e.g. <c>"0RsAAA=="</c>.</summary>
+    public static string From(uint rowVersion)
+        => "\"" + Convert.ToBase64String(BitConverter.GetBytes(rowVersion)) + "\"";
+
+    /// <summary>Parse an <c>If-Match</c> header value (quoted, optionally weak) to a token.</summary>
+    public static bool TryParse(string? ifMatch, out uint rowVersion)
+    {
+        rowVersion = 0;
+        if (string.IsNullOrWhiteSpace(ifMatch)) return false;
+
+        var v = ifMatch.Trim();
+        if (v.StartsWith("W/", StringComparison.OrdinalIgnoreCase)) v = v[2..].Trim();
+        v = v.Trim('"');
+
+        try
+        {
+            var bytes = Convert.FromBase64String(v);
+            if (bytes.Length != sizeof(uint)) return false;
+            rowVersion = BitConverter.ToUInt32(bytes);
+            return true;
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
+    }
+}

Backend/ERPCore/Common/Http/ETagged.cs

@@ -0,0 +1,7 @@
+namespace ERPCore.Common.Http;
+
+/// <summary>
+/// Pairs a response DTO with the aggregate's current row-version so the controller
+/// can emit an <c>ETag</c> header without the token leaking into the JSON body.
+/// </summary>
+public sealed record ETagged<T>(T Value, uint RowVersion);