docs: complete XML doc coverage (returns, summaries, inheritdoc)

Resolve all 622 issues flagged by the enhanced CommentChecker: add missing
<returns> tags (incl. the standard phrasing on non-generic Task methods),
add missing <summary> tags, and replace misused/redundant <inheritdoc/> on
members that override or implement nothing with real documentation.
Documentation-only — no behavior change; solution builds clean.

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs

@@ -31,6 +31,7 @@ public static class ServiceCollectionExtensions
     /// the singleton actor.
     /// </summary>
     /// <param name="services">The DI service collection to register notification outbox services into.</param>
+    /// <returns>The same <paramref name="services"/> instance for chaining.</returns>
     public static IServiceCollection AddNotificationOutbox(this IServiceCollection services)
     {
         ArgumentNullException.ThrowIfNull(services);