fix(477): guard media-server library sweeps against successful-but-empty fetches

A successful fetch returning zero items made existing.Except([]) flag the ENTIRE
library FileNotFound in one scan — feeding EmptyTrashHandler's permanent delete
and emptying every affected collection (dead channels). Add a shared
MediaServerReconciliationGuard that skips (and logs a Warning) the sweep when
incoming==0 while items exist, wired into the three library-level sweeps
(Television shows / Movie / OtherVideo).

An empty incoming set is indistinguishable at scan time from a mid-restore /
emptied-upstream error (both report a zero total), so this deliberately overrides
#476's degenerate "last item removed => empty incoming => flag" case. #476's
cascade still fires for partial deletions (survivors present); its characterization
test moves from an empty incoming to a survivor+removed partial-deletion case.

Tests: policy table (MediaServerReconciliationGuardTests) + per-scanner integration
proving the wiring (empty incoming + non-empty existing flags/reindexes nothing).
Proven non-vacuous by neutralizing the guard. Nested TV season/episode sweeps left
unguarded (bounded blast radius); ratio-threshold + projection-failure detection
deferred to a follow-up. docs/decisions.md updated.

Fixes #477

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Scanner/Core/Metadata/MediaServerOtherVideoLibraryScanner.cs

@@ -210,11 +210,15 @@ public abstract class MediaServerOtherVideoLibraryScanner<TConnectionParameters,
         }
 
         // trash OtherVideo that are no longer present on the media server
-        var fileNotFoundItemIds = existingOtherVideos.Keys.Except(incomingItemIds).ToList();
-        List<int> ids = await otherVideoRepository.FlagFileNotFound(library, fileNotFoundItemIds);
-        if (!await _scannerProxy.ReindexMediaItems(ids.ToArray(), cancellationToken))
+        if (MediaServerReconciliationGuard.ShouldFlagMissing(
+                _logger, library.Name, incomingItemIds.Count, existingOtherVideos.Count))
         {
-            _logger.LogWarning("Failed to reindex media items from scanner process");
+            var fileNotFoundItemIds = existingOtherVideos.Keys.Except(incomingItemIds).ToList();
+            List<int> ids = await otherVideoRepository.FlagFileNotFound(library, fileNotFoundItemIds);
+            if (!await _scannerProxy.ReindexMediaItems(ids.ToArray(), cancellationToken))
+            {
+                _logger.LogWarning("Failed to reindex media items from scanner process");
+            }
         }
 
         return Unit.Default;