Refactor media source handling and update collection options

- Updated `IMediaSourceCatalog` to support user-added media sources, allowing dynamic editing and management of sources.
- Removed the `UrlListSource` class as its functionality is now integrated into the new catalog structure.
- Enhanced `CollectOptions` to default `RequireProxy` to true, ensuring stricter handling of proxy requirements.
- Improved error handling in `ParseError` to include a `Subject` field for better context on failures.
- Adjusted dependency injection to reflect changes in media source management, removing old source registrations.
- Introduced background proxy checks to ensure a more robust proxy pool management during collection processes.

These changes streamline the media collection process and improve the overall user experience by providing clearer error reporting and more flexible source management.

tests/AvParser.Infrastructure.Tests/JsonSettingsServiceTests.cs

@@ -156,10 +156,13 @@ public sealed class JsonSettingsServiceTests : IDisposable
     }
 
     [Fact]
-    public void The_proxy_gate_setting_reaches_the_collector()
+    public void The_collector_defaults_to_proxy_only()
     {
-        new AppSettings(AllowDirectConnection: false).ToCollectOptions().RequireProxy.ShouldBeTrue();
-        new AppSettings(AllowDirectConnection: true).ToCollectOptions().RequireProxy.ShouldBeFalse();
+        // Whether a source may go direct is that source's own setting now; what the app-wide
+        // options must never do is default to the permissive answer.
+        new AppSettings()
+            .ToCollectOptions()
+            .RequireProxy.ShouldBeTrue();
     }
 
     [Fact]