Refactor database configuration and migrations for PostgreSQL support

- Updated default database configuration to use PostgreSQL instead of SQLite when no database.xml exists.
- Removed obsolete migration routines: MigrateUserDb, RemoveDuplicateExtras, ReseedFolderFlag.
- Introduced new read-only view projections for PostgreSQL: ItemPersonView, ItemProviderView, LibrarySummaryView, MediaStreamInfoView, UserPlaybackHistoryView, VideoItemView.
- Registered view mappings in PostgresDatabaseProvider to ensure proper handling of read-only views.
- Cleaned up unused SQLite cache size configuration in ConfigurationExtensions.

Co-authored-by: Copilot <copilot@github.com>

src/Jellyfin.Database/Jellyfin.Database.Implementations/Entities/Views/ItemProviderView.cs

@@ -0,0 +1,34 @@
+// <copyright file="ItemProviderView.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+namespace Jellyfin.Database.Implementations.Entities.Views;
+
+#pragma warning disable CS1591
+#pragma warning disable SA1600
+
+using System;
+
+/// <summary>
+/// Read-only projection of library."ItemProviders_v".
+/// Items with external provider IDs pivoted into named columns.
+/// </summary>
+public class ItemProviderView
+{
+    public Guid? ItemId { get; set; }
+    public string? Type { get; set; }
+    public string? Name { get; set; }
+    public string? OriginalTitle { get; set; }
+    public int? ProductionYear { get; set; }
+    public string? SeriesName { get; set; }
+    public int? EpisodeNumber { get; set; }
+    public int? SeasonNumber { get; set; }
+    public string? ImdbId { get; set; }
+    public string? TmdbId { get; set; }
+    public string? TvdbId { get; set; }
+    public string? TvRageId { get; set; }
+    public string? MusicBrainzAlbumId { get; set; }
+    public string? MusicBrainzArtistId { get; set; }
+    /// <summary>Gets or sets remaining providers as a JSON object string.</summary>
+    public string? OtherProviders { get; set; }
+}