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>

Jellyfin.Server.Implementations/Extensions/ServiceCollectionExtensions.cs

@@ -99,23 +99,13 @@ public static class ServiceCollectionExtensions
             }
             else
             {
-                // when nothing is setup via new Database configuration, fallback to SQLite with default settings.
+                // PostgreSQL-only build: default to PostgreSQL when no database.xml exists.
                 efCoreConfiguration = new DatabaseConfigurationOptions()
                 {
-                    DatabaseType = "Jellyfin-SQLite",
+                    DatabaseType = "Jellyfin-PostgreSQL",
                     LockingBehavior = DatabaseLockingBehaviorTypes.NoLock,
                     PresetConfigurations =
                     [
-                        new PresetConfigurationItem
-                        {
-                            Key = "SQLite",
-                            Value = new PresetDatabaseConfiguration
-                            {
-                                DatabaseType = "Jellyfin-SQLite",
-                                LockingBehavior = DatabaseLockingBehaviorTypes.NoLock,
-                                Description = "Default SQLite database configuration. Database stored in data directory."
-                            }
-                        },
                         new PresetConfigurationItem
                         {
                             Key = "PostgreSQL-Local",
@@ -135,7 +125,7 @@ public static class ServiceCollectionExtensions
         // Ensure DatabaseType is set (handle corrupted or empty configuration files)
         if (string.IsNullOrWhiteSpace(efCoreConfiguration.DatabaseType))
         {
-            efCoreConfiguration.DatabaseType = "Jellyfin-SQLite";
+            efCoreConfiguration.DatabaseType = "Jellyfin-PostgreSQL";
             configurationManager.SaveConfiguration("database", efCoreConfiguration);
         }