Move dotnet api into new directory

web/backend/Domain/Domain.Exceptions/Domain.Exceptions.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+</Project>

web/backend/Domain/Domain.Exceptions/Exceptions.cs

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+namespace Domain.Exceptions;
+
+/// <summary>
+/// Exception thrown when a resource conflict occurs (e.g., duplicate username, email already in use).
+/// Maps to HTTP 409 Conflict.
+/// </summary>
+public class ConflictException(string message) : Exception(message);
+
+/// <summary>
+/// Exception thrown when a requested resource is not found.
+/// Maps to HTTP 404 Not Found.
+/// </summary>
+public class NotFoundException(string message) : Exception(message);
+
+// Domain.Exceptions/UnauthorizedException.cs
+
+/// <summary>
+/// Exception thrown when authentication fails or is required.
+/// Maps to HTTP 401 Unauthorized.
+/// </summary>
+public class UnauthorizedException(string message) : Exception(message);
+
+/// <summary>
+/// Exception thrown when a user is authenticated but lacks permission to access a resource.
+/// Maps to HTTP 403 Forbidden.
+/// </summary>
+public class ForbiddenException(string message) : Exception(message);
+
+/// <summary>
+/// Exception thrown when business rule validation fails (distinct from FluentValidation).
+/// Maps to HTTP 400 Bad Request.
+/// </summary>
+public class ValidationException(string message) : Exception(message);