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.Infrastructure/Metadata/OmdbMetadataProvider.cs

@@ -87,6 +87,25 @@ public sealed class OmdbMetadataProvider(
         );
     }
 
+    public async Task<int?> GetSeasonEpisodeCountAsync(
+        string externalId,
+        int season,
+        CancellationToken cancellationToken
+    )
+    {
+        var url =
+            $"{_omdb.BaseUrl}/?apikey={_omdb.ApiKey}&i={Uri.EscapeDataString(externalId)}&Season={season}";
+        using var doc = await TryGetJsonAsync(url, cancellationToken);
+        if (
+            doc is null
+            || !IsResponseTrue(doc.RootElement)
+            || !doc.RootElement.TryGetProperty("Episodes", out var episodes)
+            || episodes.ValueKind != JsonValueKind.Array
+        )
+            return null;
+        return episodes.GetArrayLength();
+    }
+
     /// <summary>GET+parse; бросает при не-2xx/сетевой ошибке (для поиска — чтобы показать сбой).</summary>
     private async Task<JsonDocument> GetJsonAsync(string url, CancellationToken cancellationToken)
     {