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.StreamingGateway.Tests/StreamingGateway.Tests.csproj

@@ -1,6 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
+    <AssemblyName>Musify.StreamingGateway.Tests</AssemblyName>
+    <RootNamespace>Musify.StreamingGateway.Tests</RootNamespace>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <OutputType>Exe</OutputType>
@@ -20,7 +22,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\Hosts\Musify.StreamingGateway\Musify.StreamingGateway.csproj" />
+    <ProjectReference Include="..\..\Hosts\Musify.StreamingGateway\StreamingGateway.csproj" />
   </ItemGroup>
 
 </Project>