fix(jellyfin): fall back from missing season posters

close #981

lib/media/media_item.dart

@@ -442,6 +442,14 @@ sealed class MediaItem {
     return thumbPath;
   }
 
+  /// Secondary poster path to try when [posterThumb] returns an image URL that
+  /// exists syntactically but the server cannot serve it.
+  String? posterThumbFallback({EpisodePosterMode mode = EpisodePosterMode.seriesPoster, bool mixedHubContext = false}) {
+    if (kind != MediaKind.episode || mode != EpisodePosterMode.seasonPoster) return null;
+    final fallback = grandparentThumbPath ?? thumbPath;
+    return fallback != null && fallback != posterThumb(mode: mode, mixedHubContext: mixedHubContext) ? fallback : null;
+  }
+
   /// True when the item should render in 16:9.
   /// - Clips are always 16:9.
   /// - Episodes are 16:9 in `episodeThumbnail` mode.

lib/widgets/media_card.dart

@@ -661,13 +661,15 @@ Widget _buildPosterImage(
     final shouldBlur =
         hideSpoilers && item.shouldHideSpoiler && episodePosterMode == EpisodePosterMode.episodeThumbnail;
     posterUrl = item.posterThumb(mode: episodePosterMode, mixedHubContext: mixedHubContext);
+    final posterFallbackUrl = item.posterThumbFallback(mode: episodePosterMode, mixedHubContext: mixedHubContext);
+    final mediaClient = isOffline ? null : context.tryGetMediaClientWithFallback(item.serverId);
 
     Widget image;
 
     // Use thumb image type for 16:9 content (episodes, or movies in mixed hubs)
     if (item.usesWideAspectRatio(episodePosterMode, mixedHubContext: mixedHubContext)) {
       image = OptimizedMediaImage.thumb(
-        client: isOffline ? null : context.tryGetMediaClientWithFallback(item.serverId),
+        client: mediaClient,
         imagePath: posterUrl,
         width: knownWidth ?? double.infinity,
         height: knownHeight ?? double.infinity,
@@ -676,11 +678,20 @@ Widget _buildPosterImage(
       );
     } else {
       image = OptimizedMediaImage.poster(
-        client: isOffline ? null : context.tryGetMediaClientWithFallback(item.serverId),
+        client: mediaClient,
         imagePath: posterUrl,
         width: knownWidth ?? double.infinity,
         height: knownHeight ?? double.infinity,
         fit: BoxFit.cover,
+        errorWidget: posterFallbackUrl == null
+            ? null
+            : (_, _, _) => OptimizedMediaImage.poster(
+                client: mediaClient,
+                imagePath: posterFallbackUrl,
+                width: knownWidth ?? double.infinity,
+                height: knownHeight ?? double.infinity,
+                fit: BoxFit.cover,
+              ),
         localFilePath: localPosterPath,
       );
     }

lib/widgets/optimized_media_image.dart

@@ -319,6 +319,9 @@ class OptimizedMediaImage extends StatelessWidget {
           _imageFailureCount = 0;
           _lastFailureLog = now;
         }
+        if (errorWidget != null) {
+          return errorWidget!(context, imageUrl, error);
+        }
         return _buildErrorWidget(context, error);
       },
       frameBuilder: (context, child, frame, wasSynchronouslyLoaded) {

test/services/jellyfin_mappers_test.dart

@@ -3,6 +3,7 @@ import 'package:plezy/media/media_backend.dart';
 import 'package:plezy/media/media_kind.dart';
 import 'package:plezy/media/media_stream.dart';
 import 'package:plezy/services/jellyfin_mappers.dart';
+import 'package:plezy/services/settings_service.dart' show EpisodePosterMode;
 
 const _serverId = 'jf-machine-1';
 
@@ -123,6 +124,29 @@ void main() {
       expect(item.grandparentArtPath, '/Items/series-1/Images/Backdrop/0');
     });
 
+    test('episode season poster falls back to series poster when season image tag is absent', () {
+      final json = {
+        'Id': 'ep1',
+        'Name': 'Pilot',
+        'Type': 'Episode',
+        'SeriesId': 'series-1',
+        'SeriesName': 'Breaking Bad',
+        'SeriesPrimaryImageTag': 'seriesPrimary',
+        'SeasonId': 'season-1',
+        'SeasonName': 'Season 1',
+      };
+
+      final item = JellyfinMappers.mediaItem(json, serverId: _serverId, absolutizer: null)!;
+
+      expect(item.parentThumbPath, '/Items/season-1/Images/Primary');
+      expect(item.grandparentThumbPath, '/Items/series-1/Images/Primary?tag=seriesPrimary');
+      expect(item.posterThumb(mode: EpisodePosterMode.seasonPoster), '/Items/season-1/Images/Primary');
+      expect(
+        item.posterThumbFallback(mode: EpisodePosterMode.seasonPoster),
+        '/Items/series-1/Images/Primary?tag=seriesPrimary',
+      );
+    });
+
     test('series viewedLeafCount derived from total - UnplayedItemCount', () {
       final json = {
         'Id': 's1',