Shorten backend project file names

Each .csproj is renamed to just the project's own name within its folder
(Core/Musify.Application/Musify.Application.csproj -> Application.csproj,
and so on for every Core/Hosts/Tests project) — the Musify.* folder name
already carries that information, so the file name was redundant. Explicit
<AssemblyName>/<RootNamespace> keep the compiled assembly and C# namespaces
unchanged, so this is a build-file-only rename, not a namespace migration.
Dockerfiles, Musify.slnx, and every ProjectReference are updated to match.

backend/Tests/Musify.Application.Tests/Application.Tests.csproj

@@ -0,0 +1,32 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <AssemblyName>Musify.Application.Tests</AssemblyName>
+    <RootNamespace>Musify.Application.Tests</RootNamespace>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" />
+    <PackageReference Include="SQLitePCLRaw.bundle_e_sqlite3" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="NSubstitute" />
+    <PackageReference Include="xunit.v3" />
+    <PackageReference Include="xunit.runner.visualstudio">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\Core\Musify.Domain\Domain.csproj" />
+    <ProjectReference Include="..\..\Core\Musify.Application\Application.csproj" />
+  </ItemGroup>
+
+</Project>