feat(config): add playlist season settings and persistent season map

Adds the configuration surface for grouping seasons by TubeArchivist
playlist. No behaviour yet; the settings are inert until the providers
land.

- SortSeasonsByPlaylist toggles the feature.
- PlaylistSeasonMap persists playlist to season number associations so
  numbers stay stable across restarts and TubeArchivist outages.
- SeasonFetcherAutoEnabled records the one time library migration.
- NumberingScheme gains PlaylistIndex. Both serializers persist enums by
  name, so appending a member is safe.
- Constants gains the reserved Unsorted season (9000). Season 0 is never
  assigned because Jellyfin force-renames it to "Specials".

PlaylistSeasonMap carries [JsonInclude] deliberately. Jellyfin stores
plugin configuration as XML, which round-trips a private setter via the
ICollection add pattern, but the configuration API endpoint deserializes
with System.Text.Json, which ignores non-public setters. Without the
attribute the collection is dropped on every configuration page save and
BasePlugin.UpdateConfiguration writes the emptied collection back to
disk. A public setter is not an alternative: CA2227 is enabled and
TreatWarningsAsErrors would fail the build.

Jellyfin.Plugin.TubeArchivistMetadata/Configuration/NumberingScheme.cs

@@ -14,4 +14,10 @@ public enum NumberingScheme
     /// YYYYMMDD (e.g. 20250804 for August 4th, 2025).
     /// </summary>
     YYYYMMDD,
+
+    /// <summary>
+    /// The video position within its TubeArchivist playlist.
+    /// Falls back to <see cref="Default"/> when the video does not belong to a playlist.
+    /// </summary>
+    PlaylistIndex,
 }

Jellyfin.Plugin.TubeArchivistMetadata/Configuration/PlaylistSeasonMapEntry.cs

@@ -0,0 +1,47 @@
+namespace Jellyfin.Plugin.TubeArchivistMetadata.Configuration
+{
+    /// <summary>
+    /// A persisted association between a TubeArchivist playlist and the Jellyfin season number
+    /// used to represent it.
+    /// </summary>
+    /// <remarks>
+    /// Jellyfin groups episodes into seasons using <c>ParentIndexNumber</c>, which is an integer,
+    /// while TubeArchivist identifies playlists by string id. This entry persists the mapping so
+    /// season numbers stay stable across restarts and metadata refreshes.
+    /// A plain dictionary is not used because Jellyfin's XML configuration serializer does not
+    /// reliably round-trip dictionary types.
+    /// </remarks>
+    public class PlaylistSeasonMapEntry
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PlaylistSeasonMapEntry"/> class.
+        /// </summary>
+        /// <remarks>
+        /// A parameterless constructor is required by the XML serializer used to persist the
+        /// plugin configuration.
+        /// </remarks>
+        public PlaylistSeasonMapEntry()
+        {
+            PlaylistId = string.Empty;
+            PlaylistName = string.Empty;
+        }
+
+        /// <summary>
+        /// Gets or sets the TubeArchivist playlist id.
+        /// </summary>
+        public string PlaylistId { get; set; }
+
+        /// <summary>
+        /// Gets or sets the Jellyfin season number representing the playlist.
+        /// </summary>
+        public int SeasonNumber { get; set; }
+
+        /// <summary>
+        /// Gets or sets the last known TubeArchivist playlist name.
+        /// </summary>
+        /// <remarks>
+        /// Stored so seasons keep a usable label when TubeArchivist is unreachable.
+        /// </remarks>
+        public string PlaylistName { get; set; }
+    }
+}

Jellyfin.Plugin.TubeArchivistMetadata/Configuration/PluginConfiguration.cs

@@ -1,7 +1,9 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Data;
 using System.Linq;
+using System.Text.Json.Serialization;
 using Jellyfin.Plugin.TubeArchivistMetadata.Utilities;
 using MediaBrowser.Model.Plugins;
 using Microsoft.Extensions.Logging;
@@ -47,6 +49,9 @@ namespace Jellyfin.Plugin.TubeArchivistMetadata.Configuration
             TAJFProgressTaskInterval = 60;
             JFTAPlaylistsSyncTaskInterval = 60;
             TAJFPlaylistsSyncTaskInterval = 60;
+            SortSeasonsByPlaylist = false;
+            SeasonFetcherAutoEnabled = false;
+            PlaylistSeasonMap = new Collection<PlaylistSeasonMapEntry>();
         }
 
         /// <summary>
@@ -191,6 +196,49 @@ namespace Jellyfin.Plugin.TubeArchivistMetadata.Configuration
         /// </summary>
         public NumberingScheme EpisodeNumberingScheme { get; set; } = NumberingScheme.Default;
 
+        /// <summary>
+        /// Gets or sets a value indicating whether to group episodes into seasons by TubeArchivist
+        /// playlist instead of by upload year.
+        /// </summary>
+        /// <remarks>
+        /// Changing this setting requires a "Refresh metadata" with "Replace all metadata" enabled:
+        /// Jellyfin only overwrites an existing season name when replacing metadata.
+        /// </remarks>
+        public bool SortSeasonsByPlaylist { get; set; }
+
+        /// <summary>
+        /// Gets the persisted TubeArchivist playlist to Jellyfin season number associations.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// Populated automatically; not user editable. Entries are append-only so season numbers
+        /// remain stable once assigned.
+        /// </para>
+        /// <para>
+        /// <see cref="JsonIncludeAttribute"/> is required and must not be removed. Jellyfin persists
+        /// plugin configuration as XML, but the plugin configuration API endpoint deserializes the
+        /// posted body with System.Text.Json, which ignores non-public setters. Without this
+        /// attribute the collection is dropped whenever the configuration page is saved, and
+        /// <c>BasePlugin.UpdateConfiguration</c> then writes the emptied collection back to disk.
+        /// A public setter is not an option because it violates CA2227.
+        /// This is unrelated to the Newtonsoft usage of the TubeArchivist API models.
+        /// </para>
+        /// </remarks>
+        [JsonInclude]
+        public Collection<PlaylistSeasonMapEntry> PlaylistSeasonMap { get; private set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the one time migration which enables this
+        /// plugin's Season metadata fetcher on existing libraries has already run.
+        /// </summary>
+        /// <remarks>
+        /// Libraries created before the Season provider existed store an empty Season fetcher list,
+        /// which Jellyfin treats as "all disabled" rather than "use the defaults". The migration
+        /// repairs that once; keeping this flag means a fetcher removed by hand afterwards stays
+        /// removed instead of being re-added on every restart.
+        /// </remarks>
+        public bool SeasonFetcherAutoEnabled { get; set; }
+
         /// <summary>
         /// Gets the playback progress owners Jellyfin usernames to synchronize data from TubeArchivist.
         /// </summary>

Jellyfin.Plugin.TubeArchivistMetadata/Constants.cs

@@ -19,5 +19,19 @@ namespace Jellyfin.Plugin.TubeArchivistMetadata
         /// Providers name.
         /// </summary>
         public const string ProviderName = "TubeArchivist";
+
+        /// <summary>
+        /// Season number used to group videos which do not belong to any TubeArchivist playlist.
+        /// </summary>
+        /// <remarks>
+        /// Season 0 cannot be used: Jellyfin's <c>SeasonMetadataService.BeforeSaveInternal</c>
+        /// force-renames season 0 to the library's "Specials" display name.
+        /// </remarks>
+        public const int UnsortedSeasonNumber = 9000;
+
+        /// <summary>
+        /// Display name of the season grouping videos which do not belong to any TubeArchivist playlist.
+        /// </summary>
+        public const string UnsortedSeasonName = "Unsorted";
     }
 }