PostgreSQL: Production fixes—UPSERT, remote backup, auth logs

- Fix: Atomic UPSERT for BaseItemProviders (resolves duplicate key errors during concurrent metadata refresh; clears navigation property to prevent EF Core tracking conflicts)
- Add: Remote PostgreSQL backup support (removes localhost-only restriction; works with pg_dump/pg_restore for both local and remote DBs)
- Add: Configurable backup disable option (`disable-backups`)
- Fix: Query timeout and performance (documented `command-timeout` config, added performance index scripts)
- Fix: Authentication errors now log as warnings with clear messages (ExceptionMiddleware), reducing log noise
- Fix: SyncPlay authorization handler validates user before lookup, logs warnings for unauthenticated/unknown users (returns 403/404)
- Fix: Database deadlock detection logs warnings and allows EF Core auto-retry
- Add: Configurable LibraryMonitorDelay (min 30s, default 60s)
- Fix: SQLite migration filtering—skip SQLite-only migrations on PostgreSQL
- Chore: Suppress StyleCop warnings (SA1137, etc.) for project consistency
- Docs: 21 documentation files added/updated (config, backup, performance, troubleshooting, session summary)
- All changes are backward compatible and production-ready

Jellyfin.Server/Migrations/JellyfinMigrationService.cs

@@ -403,7 +403,7 @@ internal class JellyfinMigrationService
 
         backupInstruction = Migrations.SelectMany(e => e)
            .Where(e => appliedMigrations.All(f => f.MigrationId != e.BuildCodeMigrationId()))
-           .Where(e => isSqliteProvider || !e.Metadata.RequiresSqlite)  // Skip SQLite migrations for non-SQLite providers
+           .Where(e => isSqliteProvider || !e.Metadata.RequiresSqlite) // Skip SQLite migrations for non-SQLite providers
            .Select(e => e.BackupRequirements)
            .Where(e => e is not null)
            .Aggregate(backupInstruction, MergeBackupAttributes!);