fix: better forward season merging logic

Added better forward merging logic for the season merging to allow the "primary" series to match the year regex. Also uglified the year regex some more.

**Note**: Casual reminder not to use the season merging _advanced setting_ if you don't like pain.

Shokofin/API/ShokoAPIManager.cs

@@ -21,7 +21,7 @@ namespace Shokofin.API;
 public class ShokoAPIManager : IDisposable
 {
     // Note: This regex will only get uglier with time.
-    private static readonly Regex YearRegex = new(@"\s+\((?<year>\d{4})(?:dai [2-9] bu)?\)\s*$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+    private static readonly Regex YearRegex = new(@"\s+\((?<year>\d{4})(?: dai [2-9] bu)?\)\s*$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
 
     private readonly ILogger<ShokoAPIManager> Logger;
 
@@ -860,11 +860,12 @@ private async Task<SeasonInfo> CreateSeasonInfo(Series series)
                 var maxDaysThreshold = config.EXPERIMENTAL_MergeSeasonsMergeWindowInDays;
                 if (result.Success)
                 {
+                    Series? lastPrequelSeries = null;
                     var adjustedMainTitle = mainTitle[..^result.Length];
                     var currentDate = series.AniDBEntity.AirDate.Value;
                     var currentRelations = relations;
                     while (currentRelations.Count > 0) {
-                        foreach (var prequelRelation in currentRelations.Where(relation => relation.Type == RelationType.Prequel && relation.RelatedIDs.Shoko.HasValue)) {
+                        foreach (var prequelRelation in currentRelations.Where(relation => relation.Type is RelationType.Prequel && relation.RelatedIDs.Shoko.HasValue)) {
                             var prequelSeries = await APIClient.GetSeries(prequelRelation.RelatedIDs.Shoko!.Value.ToString());
                             if (prequelSeries.IDs.ParentGroup != series.IDs.ParentGroup)
                                 continue;
@@ -897,6 +898,7 @@ private async Task<SeasonInfo> CreateSeasonInfo(Series series)
 
                             var adjustedPrequelMainTitle = prequelMainTitle[..^prequelResult.Length];
                             if (string.Equals(adjustedMainTitle, adjustedPrequelMainTitle, StringComparison.InvariantCultureIgnoreCase)) {
+                                lastPrequelSeries = prequelSeries;
                                 currentDate = prequelDate;
                                 currentRelations = await APIClient.GetSeriesRelations(prequelSeries.IDs.Shoko.ToString()).ConfigureAwait(false);
                                 goto continuePrequelWhileLoop;
@@ -905,8 +907,55 @@ private async Task<SeasonInfo> CreateSeasonInfo(Series series)
                         breakPrequelWhileLoop: break;
                         continuePrequelWhileLoop: continue;
                     }
+                    // Although we didn't find a _pure_ "primary" season candidate, we found a series with a matching year regex pattern, so get the series ids for that.
+                    if (lastPrequelSeries is not null) {
+                        (primaryId, extraIds) = await GetSeriesIdsForSeason(lastPrequelSeries);
+                    }
+                    // We potentially have an _unpure_ "primary" season candidate, so look for any "follow-up" season candidates.
+                    else if (extraIds.Count is 0) {
+                        currentDate = series.AniDBEntity.AirDate.Value;
+                        currentRelations = relations;
+                        while (currentRelations.Count > 0) {
+                            foreach (var sequelRelation in currentRelations.Where(relation => relation.Type == RelationType.Sequel && relation.RelatedIDs.Shoko.HasValue)) {
+                                var sequelSeries = await APIClient.GetSeries(sequelRelation.RelatedIDs.Shoko!.Value.ToString());
+                                if (sequelSeries.IDs.ParentGroup != series.IDs.ParentGroup)
+                                    continue;
+
+                                if (!config.EXPERIMENTAL_MergeSeasonsTypes.Contains(await GetCustomSeriesType(sequelSeries.IDs.Shoko.ToString()) ?? sequelSeries.AniDBEntity.Type))
+                                    continue;
+
+                                if (sequelSeries.AniDBEntity.AirDate is null)
+                                    continue;
+
+                                var sequelDate = sequelSeries.AniDBEntity.AirDate.Value;
+                                if (sequelDate < currentDate)
+                                    continue;
+
+                                if (maxDaysThreshold > 0) {
+                                    var deltaDays = (int)Math.Floor((sequelDate - currentDate).TotalDays);
+                                    if (deltaDays > maxDaysThreshold)
+                                        continue;
+                                }
+
+                                var sequelMainTitle = sequelSeries.AniDBEntity.Titles.First(title => title.Type == TitleType.Main).Value;
+                                var sequelResult = YearRegex.Match(sequelMainTitle);
+                                if (!sequelResult.Success)
+                                    continue;
+
+                                var adjustedSequelMainTitle = sequelMainTitle[..^sequelResult.Length];
+                                if (string.Equals(adjustedMainTitle, adjustedSequelMainTitle, StringComparison.InvariantCultureIgnoreCase)) {
+                                    extraIds.Add(sequelSeries.IDs.Shoko.ToString());
+                                    currentDate = sequelDate;
+                                    currentRelations = await APIClient.GetSeriesRelations(sequelSeries.IDs.Shoko.ToString()).ConfigureAwait(false);
+                                    goto continueSequelWhileLoop;
+                                }
+                            }
+                            break;
+                            continueSequelWhileLoop: continue;
+                        }
+                    }
                 }
-                // We potentially have a "primary" season candidate, so look for any "follow-up" season candidates.
+                // We potentially have a _pure_ "primary" season candidate, so look for any "follow-up" season candidates.
                 else {
                     var currentDate = series.AniDBEntity.AirDate.Value;
                     var adjustedMainTitle = mainTitle;