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/Core/Musify.Application/Application.csproj

@@ -0,0 +1,30 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <AssemblyName>Musify.Application</AssemblyName>
+    <RootNamespace>Musify.Application</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="ErrorOr" />
+    <PackageReference Include="MassTransit" />
+    <PackageReference Include="Mediator.Abstractions" />
+    <PackageReference Include="Mediator.SourceGenerator">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.EntityFrameworkCore" />
+    <PackageReference Include="Microsoft.Extensions.Configuration" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" />
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" />
+    <PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" />
+    <PackageReference Include="MimeMapping" />
+    <PackageReference Include="X.PagedList" />
+    <PackageReference Include="X.PagedList.EF" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Musify.Domain\Domain.csproj" />
+  </ItemGroup>
+
+</Project>