refactor: central package management, shared build props, security fix

- Add Directory.Packages.props (CPM): all package versions centralized,
  remove Version attributes from every PackageReference
- Add Directory.Build.props: shared TargetFramework/Nullable/ImplicitUsings
- Drop redundant per-project RootNamespace/AssemblyName (default = project name)
  and stale NuGet metadata (PackageId/Version/Authors/RepositoryUrl)
- Bump Microsoft.OpenApi to 2.9.0 (CVE-2026-49451, was transitive 2.0.0);
  add explicit reference in Musify.Api which uses it directly
- Add repo-scoped nuget.config pinning nuget.org for reproducible restore

Build: 0 warnings, 0 errors.

nuget.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+</configuration>