refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Infrastructure/Persistence/DatabaseDependencyInjection.cs

@@ -0,0 +1,27 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
+using Musify.Application.Contracts;
+using Musify.Infrastructure.Configuration;
+
+namespace Musify.Infrastructure.Persistence
+{
+    public static class DatabaseDependencyInjection
+    {
+        public static IServiceCollection AddDatabase(this IServiceCollection serviceDescriptors, IConfiguration configuration)
+        {
+            serviceDescriptors
+                .AddOptionsWithValidateOnStart<DatabaseConfiguration>()
+                .Bind(configuration.GetRequiredSection(DatabaseConfiguration.SectionName))
+                .ValidateDataAnnotations();
+
+            serviceDescriptors.AddSingleton(serviceProvider =>
+                serviceProvider.GetRequiredService<IOptions<DatabaseConfiguration>>().Value);
+
+            serviceDescriptors.AddDbContext<Database>();
+            serviceDescriptors.AddScoped<IDatabase>(serviceProvider => serviceProvider.GetRequiredService<Database>());
+
+            return serviceDescriptors;
+        }
+    }
+}