refactor(infra): register auditable interceptor via DI

Register AuditableEntityInterceptor as a singleton and attach it through the
AddDbContext options callback instead of newing it up inside
Database.OnConfiguring. The interceptor is stateless, so a single shared
instance is reused across contexts and its wiring lives in the DI layer.
The design-time factory keeps working (interceptor is runtime-only, irrelevant
for migrations).

Verified: build + ef migrations list (design-time context) OK; the Worker host
builds and validates the full DI graph (reaches endpoint configuration) with no
captive-dependency or interceptor error.

Build: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Infrastructure/Persistence/DatabaseDependencyInjection.cs

@@ -1,3 +1,4 @@
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Options;
@@ -18,7 +19,11 @@ namespace Musify.Infrastructure.Persistence
             serviceDescriptors.AddSingleton(serviceProvider =>
                 serviceProvider.GetRequiredService<IOptions<DatabaseConfiguration>>().Value);
 
-            serviceDescriptors.AddDbContext<Database>();
+            serviceDescriptors.AddSingleton<AuditableEntityInterceptor>();
+
+            serviceDescriptors.AddDbContext<Database>((serviceProvider, options) =>
+                options.AddInterceptors(serviceProvider.GetRequiredService<AuditableEntityInterceptor>()));
+
             serviceDescriptors.AddScoped<IDatabase>(serviceProvider => serviceProvider.GetRequiredService<Database>());
 
             return serviceDescriptors;