refactor: restructure solution to match ItemTracker layered architecture

- Remove Core (replaced by dedicated layers)
- Add DAL.Abstract (domain model + repository interfaces)
- Add DAL.EF (EF context, entities, migrations — references DAL.Abstract)
- Add Services (business logic — references DAL.Abstract only, not DAL.EF)
- Api and Worker reference Services + DAL.EF for DI wiring
- Replace Core.Tests with Services.Tests
- Dependency boundary enforced: Services never references DAL.EF

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Services.Tests/HrynCo.NotificationService.Services.Tests.csproj

@@ -0,0 +1,26 @@
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+
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+    <ImplicitUsings>enable</ImplicitUsings>
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+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
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+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\HrynCo.NotificationService.Services\HrynCo.NotificationService.Services.csproj" />
+    <ProjectReference Include="..\HrynCo.NotificationService.DAL.Abstract\HrynCo.NotificationService.DAL.Abstract.csproj" />
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