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/Database.cs

@@ -15,7 +15,6 @@ namespace Musify.Infrastructure.Persistence
         protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
         {
             optionsBuilder.UseNpgsql(configuration.ConnectionString);
-            optionsBuilder.AddInterceptors(new AuditableEntityInterceptor());
         }
 
         protected override void OnModelCreating(ModelBuilder modelBuilder)