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.Api/Middleware/ExceptionMiddleware.cs

@@ -76,13 +76,27 @@ public class ExceptionMiddleware
                 || ex is AuthenticationException
                 || ex is FileNotFoundException;
 
+            // Authentication failures are expected and don't need ERROR level logging
+            bool isAuthenticationError = ex is SecurityException || ex is AuthenticationException;
+
             if (ignoreStackTrace)
             {
-                _logger.LogError(
-                    "Error processing request: {ExceptionMessage}. URL {Method} {Url}.",
-                    ex.Message.TrimEnd('.'),
-                    context.Request.Method,
-                    context.Request.Path);
+                if (isAuthenticationError)
+                {
+                    // Log authentication errors as warnings with user-friendly message
+                    _logger.LogWarning(
+                        "Access denied: Unable to process request. Authentication token missing or invalid. URL {Method} {Url}.",
+                        context.Request.Method,
+                        context.Request.Path);
+                }
+                else
+                {
+                    _logger.LogError(
+                        "Error processing request: {ExceptionMessage}. URL {Method} {Url}.",
+                        ex.Message.TrimEnd('.'),
+                        context.Request.Method,
+                        context.Request.Path);
+                }
             }
             else
             {