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

@@ -1,5 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
+  <PropertyGroup>
+    <AssemblyName>Musify.Application</AssemblyName>
+    <RootNamespace>Musify.Application</RootNamespace>
+  </PropertyGroup>
+
   <ItemGroup>
     <PackageReference Include="ErrorOr" />
     <PackageReference Include="MassTransit" />
@@ -19,7 +24,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\Musify.Domain\Musify.Domain.csproj" />
+    <ProjectReference Include="..\Musify.Domain\Domain.csproj" />
   </ItemGroup>
 
 </Project>