Add missing episodes feature: implement FindMissingEpisodes endpoint, update metadata providers to retrieve season episode counts, and enhance frontend components for displaying missing episodes report. Update translations for new UI elements.

backend/src/TeleWave.Application/Common/Interfaces/IMetadataProvider.cs

@@ -21,6 +21,13 @@ public interface IMetadataProvider
         int episode,
         CancellationToken cancellationToken
     );
+
+    /// <summary>Сколько серий в указанном сезоне по данным источника (null — сезон не найден/нет данных).</summary>
+    Task<int?> GetSeasonEpisodeCountAsync(
+        string externalId,
+        int season,
+        CancellationToken cancellationToken
+    );
 }
 
 /// <summary>Резолвит провайдер по ключу и перечисляет реально настроенные (с API-ключом) источники.</summary>

backend/src/TeleWave.Application/Metadata/FindMissingEpisodes/FindMissingEpisodesQuery.cs

@@ -0,0 +1,20 @@
+using LiteCqrs;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Metadata.FindMissingEpisodes;
+
+/// <summary>Отчёт: каких серий не хватает в загруженных сезонах шоу (по данным привязанного источника).</summary>
+public sealed record FindMissingEpisodesQuery(Guid ShowId) : IQuery<Result<MissingEpisodesReport>>;
+
+public sealed record MissingEpisodesReport(IReadOnlyList<SeasonGapDto> Seasons);
+
+/// <param name="Season">Номер сезона (есть хотя бы одна загруженная серия).</param>
+/// <param name="Expected">Сколько серий в сезоне по источнику (null — источник не отдал данные).</param>
+/// <param name="Loaded">Сколько серий этого сезона загружено.</param>
+/// <param name="Missing">Отсутствующие номера серий (пусто — все на месте либо Expected неизвестен).</param>
+public sealed record SeasonGapDto(
+    int Season,
+    int? Expected,
+    int Loaded,
+    IReadOnlyList<int> Missing
+);

backend/src/TeleWave.Application/Metadata/FindMissingEpisodes/FindMissingEpisodesQueryHandler.cs

@@ -0,0 +1,78 @@
+using LiteCqrs;
+using Microsoft.EntityFrameworkCore;
+using TeleWave.Application.Common.Interfaces;
+using TeleWave.Application.Common.Models;
+using TeleWave.Application.Library;
+
+namespace TeleWave.Application.Metadata.FindMissingEpisodes;
+
+public sealed class FindMissingEpisodesQueryHandler(
+    IAppDbContext dbContext,
+    IMetadataProviderResolver resolver
+) : IQueryHandler<FindMissingEpisodesQuery, Result<MissingEpisodesReport>>
+{
+    public async Task<Result<MissingEpisodesReport>> Handle(
+        FindMissingEpisodesQuery query,
+        CancellationToken cancellationToken
+    )
+    {
+        var show = await dbContext
+            .Shows.AsNoTracking()
+            .Include(s => s.Episodes)
+            .FirstOrDefaultAsync(s => s.Id == query.ShowId, cancellationToken);
+        if (show is null)
+            return Result.Failure<MissingEpisodesReport>(ShowErrors.NotFound);
+
+        if (string.IsNullOrEmpty(show.MetadataExternalId))
+            return Result.Failure<MissingEpisodesReport>(MetadataErrors.NoLinkedSource);
+
+        var provider = show.MetadataProvider is { } key ? resolver.Resolve(key) : null;
+        if (provider is null)
+            return Result.Failure<MissingEpisodesReport>(MetadataErrors.ProviderNotAvailable);
+
+        // Имена файлов — чтобы распознать номера у серий, где они ещё не проставлены.
+        var assetIds = show.Episodes.Select(e => e.MediaAssetId).Distinct().ToList();
+        var names = await dbContext
+            .MediaAssets.AsNoTracking()
+            .Where(a => assetIds.Contains(a.Id))
+            .Select(a => new { a.Id, a.OriginalFileName })
+            .ToDictionaryAsync(a => a.Id, a => a.OriginalFileName, cancellationToken);
+
+        // Загруженные номера серий по сезонам (только сезоны с хотя бы одной серией).
+        var loadedBySeason = new Dictionary<int, HashSet<int>>();
+        foreach (var episode in show.Episodes)
+        {
+            var season = episode.Season;
+            var number = episode.Episode;
+            if ((season is null || number is null) && names.TryGetValue(episode.MediaAssetId, out var name))
+            {
+                if (EpisodeName.Parse(name) is { } parsed)
+                    (season, number) = (parsed.Season, parsed.Episode);
+            }
+            if (season is null || number is null)
+                continue;
+
+            if (!loadedBySeason.TryGetValue(season.Value, out var set))
+                loadedBySeason[season.Value] = set = [];
+            set.Add(number.Value);
+        }
+
+        var seasons = new List<SeasonGapDto>();
+        foreach (var season in loadedBySeason.Keys.OrderBy(s => s))
+        {
+            var loaded = loadedBySeason[season];
+            var expected = await provider.GetSeasonEpisodeCountAsync(
+                show.MetadataExternalId,
+                season,
+                cancellationToken
+            );
+            var missing =
+                expected is { } exp
+                    ? Enumerable.Range(1, exp).Where(n => !loaded.Contains(n)).ToList()
+                    : [];
+            seasons.Add(new SeasonGapDto(season, expected, loaded.Count, missing));
+        }
+
+        return Result.Success(new MissingEpisodesReport(seasons));
+    }
+}