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/LibrarySummaryView.cs

@@ -0,0 +1,33 @@
+// <copyright file="LibrarySummaryView.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+namespace Jellyfin.Database.Implementations.Entities.Views;
+
+#pragma warning disable CS1591
+#pragma warning disable SA1600
+
+/// <summary>
+/// Read-only projection of the library."LibrarySummary_v" view.
+/// One row per item type with aggregate statistics across the whole library.
+/// </summary>
+public class LibrarySummaryView
+{
+    public string? Type { get; set; }
+    public long? ItemCount { get; set; }
+    public long? WithYear { get; set; }
+    public int? OldestYear { get; set; }
+    public int? NewestYear { get; set; }
+    public decimal? AvgCommunityRating { get; set; }
+    public decimal? TotalRunTimeTicks { get; set; }
+    public decimal? TotalRuntimeHours { get; set; }
+    public decimal? TotalSizeBytes { get; set; }
+    public decimal? TotalSizeGB { get; set; }
+    public long? Count4K { get; set; }
+    public long? Count1080p { get; set; }
+    public long? Count720p { get; set; }
+    public long? CountSD { get; set; }
+    public long? CountDolbyVision { get; set; }
+    public long? CountHDR10Plus { get; set; }
+    public long? CountHDR10 { get; set; }
+}