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.SiteEventLogging/EventLogPurgeService.cs

@@ -17,6 +17,7 @@ namespace ZB.MOM.WW.ScadaBridge.SiteEventLogging;
 /// active" so non-clustered hosts and unit tests are unaffected — backward
 /// compatible with the prior "run on every host" behaviour.
 /// </summary>
+/// <returns><c>true</c> if this node is the active site member and should run the purge; <c>false</c> to skip.</returns>
 public delegate bool SiteEventLogActiveNodeCheck();
 
 /// <summary>
@@ -200,6 +201,7 @@ public class EventLogPurgeService : BackgroundService
     /// the raw file size from <c>page_count</c>) means the storage-cap loop observes
     /// space being reclaimed even if free pages have not yet been returned to the OS.
     /// </summary>
+    /// <returns>The logical database size in bytes (used pages × page size).</returns>
     internal long GetDatabaseSizeBytes()
     {
         return _eventLogger.WithConnection(connection =>