first commit

Backend/ERPCore/System/Errors/DomainException.cs

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+namespace ERPCore.System.Errors;
+
+/// <summary>
+/// Base for domain / business-rule violations. Carries a stable <see cref="Code"/>
+/// (see the error catalog in docs/11-BACKEND-PHASE1.md) and an HTTP
+/// <see cref="StatusCode"/>; mapped to RFC 7807 ProblemDetails by
+/// <see cref="DomainExceptionHandler"/>.
+/// </summary>
+public class DomainException : Exception
+{
+    public string Code { get; }
+    public int StatusCode { get; }
+
+    public DomainException(string code, string message, int statusCode = 400)
+        : base(message)
+    {
+        Code = code;
+        StatusCode = statusCode;
+    }
+}
+
+/// <summary>404 — a referenced resource does not exist.</summary>
+public sealed class NotFoundException : DomainException
+{
+    public NotFoundException(string message) : base(ErrorCodes.NotFound, message, 404) { }
+}
+
+/// <summary>409 — the request conflicts with current server state.</summary>
+public sealed class ConflictException : DomainException
+{
+    public ConflictException(string message) : base(ErrorCodes.Conflict, message, 409) { }
+}

Backend/ERPCore/System/Errors/DomainExceptionHandler.cs

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+using Microsoft.AspNetCore.Diagnostics;
+using Microsoft.AspNetCore.Mvc;
+
+namespace ERPCore.System.Errors;
+
+/// <summary>
+/// Translates <see cref="DomainException"/> into an RFC 7807 ProblemDetails
+/// response, preserving the stable domain <c>code</c> as an extension. Non-domain
+/// exceptions are left unhandled so the framework returns a 500 ProblemDetails.
+/// Registered via AddExceptionHandler and invoked by UseExceptionHandler.
+/// </summary>
+public sealed class DomainExceptionHandler : IExceptionHandler
+{
+    private readonly IProblemDetailsService _problemDetails;
+
+    public DomainExceptionHandler(IProblemDetailsService problemDetails)
+        => _problemDetails = problemDetails;
+
+    public async ValueTask<bool> TryHandleAsync(
+        HttpContext httpContext, Exception exception, CancellationToken cancellationToken)
+    {
+        if (exception is not DomainException domain)
+            return false;
+
+        httpContext.Response.StatusCode = domain.StatusCode;
+
+        return await _problemDetails.TryWriteAsync(new ProblemDetailsContext
+        {
+            HttpContext = httpContext,
+            Exception = domain,
+            ProblemDetails = new ProblemDetails
+            {
+                Status = domain.StatusCode,
+                Title = "Domain rule violation",
+                Detail = domain.Message,
+                Extensions = { ["code"] = domain.Code }
+            }
+        });
+    }
+}

Backend/ERPCore/System/Errors/ErrorCodes.cs

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+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
+/// the two stay in sync.
+/// </summary>
+public static class ErrorCodes
+{
+    public const string Validation = "validation_error";
+    public const string NotFound = "not_found";
+    public const string Conflict = "conflict";
+}