Add Infrastructure/Api/StreamingGateway test projects

Second pass on backend/Tests/, covering what Domain+Application (the first
pass) deliberately left out — the parts that talk to real infrastructure:

  Musify.Infrastructure.Tests   Database/migrations, StorageService,
                                 StreamTicketService, AuditableEntityInterceptor,
                                 PictureService, SingleFlightCache,
                                 PlayListPresetSeeder.
  Musify.Api.Tests               Real HTTP endpoints via WebApplicationFactory
                                 (routing, auth, ValidationFilter, ErrorOr->HTTP
                                 mapping), plus isolated unit tests for
                                 ErrorOrHttpExtensions, CurrentUser and the
                                 FluentValidation validators.
  Musify.StreamingGateway.Tests  TicketValidator (RS256) and
                                 TicketValidationMiddleware (path traversal,
                                 prefix boundary, ticket extraction).

Musify.Worker gets no test project: its Program.cs is pure DI/host wiring
(Hangfire, MassTransit, jobs); the logic it runs lives in Musify.Infrastructure,
already covered there.

Per the user's direction, the parts that touch real infrastructure use
Testcontainers instead of fakes: a real Postgres (Testcontainers.PostgreSql)
for Infrastructure.Tests and Api.Tests, and — matching deploy/compose.yml
exactly (chrislusf/seaweedfs:latest, same startup command), not MinIO — a
real SeaweedFS container for StorageService. Api.Tests boots the actual
Musify.Api host against that Postgres (real routing/auth/validation) but
substitutes the services that talk over the network (storage, YouTube, the
message bus, stream-ticket signing) with NSubstitute fakes, and swaps the
JwtBearer scheme for a test-only header-based one (FakeAuthenticationHandler)
so tests don't need a reachable Zitadel. Application.Tests' approach — a real
DbContext (SQLite in-memory) rather than mocking IDatabase, since it exposes
DbSet<T> directly and handlers compose real LINQ queries against it — carries
over into Infrastructure.Tests/Api.Tests via real Postgres instead.

Testing against the real backend (not a fake) found two genuine production
bugs, both fixed here:

- StorageService.RemoveFolderAsync threw NullReferenceException against
  SeaweedFS: its DeleteObjects response omits the <Error> list entirely when
  nothing failed, which the AWS SDK deserializes as null rather than an empty
  list (real AWS S3 always returns one). Now treats null as no errors.
- AuditableEntityInterceptor never bumped UpdatedAt on a normal update (load,
  mutate a property, SaveChanges) — only on insert. SavingChanges(Async) fires
  before EF's own automatic change detection for that save, so an entity
  modified through a plain property setter still reported EntityState.Unchanged
  inside the interceptor. Now calls ChangeTracker.DetectChanges() explicitly
  first, which is the documented fix for exactly this interceptor pattern.

Also required to make the Api host testable: Program.cs now ends with `public
partial class Program;` (WebApplicationFactory<Program> needs the top-level-
statements Program class to be public — no behavior change), and
Directory.Packages.props gains Microsoft.AspNetCore.Mvc.Testing, Testcontainers
and Testcontainers.PostgreSql.

249 tests total across all five projects, all green: `dotnet test
backend/Musify.slnx` (needs Docker running for Infrastructure.Tests and
Api.Tests).

backend/Core/Musify.Infrastructure/Persistence/AuditableEntityInterceptor.cs

@@ -24,6 +24,13 @@ namespace Musify.Infrastructure.Persistence
             if (context is null)
                 return;
 
+            // SavingChanges(Async) fires before EF's own automatic change detection for this
+            // save, so entities modified through plain property assignment (query, then set a
+            // property, then SaveChanges — the normal case) still show as Unchanged here unless
+            // something already forced detection. Force it explicitly, otherwise UpdatedAt is
+            // never bumped on a real update, only on inserts.
+            context.ChangeTracker.DetectChanges();
+
             var now = DateTime.UtcNow;
 
             foreach (var entry in context.ChangeTracker.Entries<IAuditable>())

backend/Core/Musify.Infrastructure/Services/StorageService.cs

@@ -289,9 +289,13 @@ namespace Musify.Infrastructure.Services
 
                 var deleteResponse = await amazonS3.DeleteObjectsAsync(deleteRequest, cancellationToken);
 
-                if (deleteResponse.DeleteErrors.Count > 0)
+                // Some S3-compatible backends (SeaweedFS included) omit the <Error> list from the
+                // XML response entirely when nothing failed, which the SDK deserializes as null
+                // rather than an empty list — real AWS S3 always returns a (possibly empty) list.
+                var deleteErrors = deleteResponse.DeleteErrors ?? [];
+                if (deleteErrors.Count > 0)
                 {
-                    var errors = string.Join(", ", deleteResponse.DeleteErrors.Select(e => $"{e.Key}: {e.Message}"));
+                    var errors = string.Join(", ", deleteErrors.Select(e => $"{e.Key}: {e.Message}"));
                     throw new InvalidOperationException($"Failed to delete some objects in folder {folderKey}: {errors}");
                 }