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.Domain/Domain.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <AssemblyName>Musify.Domain</AssemblyName>
+    <RootNamespace>Musify.Domain</RootNamespace>
+  </PropertyGroup>
+
+</Project>

backend/Core/Musify.Domain/Musify.Domain.csproj

@@ -1,3 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-
-</Project>