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

src/Jellyfin.Database/Jellyfin.Database.Implementations/JellyfinDbContext.cs

@@ -280,6 +280,32 @@ public class JellyfinDbContext(DbContextOptions<JellyfinDbContext> options, ILog
             }).ConfigureAwait(false);
             return result;
         }
+        catch (DbUpdateException ex)
+        {
+            // Check if it's a constraint violation (works across all database providers)
+            var isConstraintViolation = ex.InnerException?.GetType().Name.Contains("Exception", StringComparison.Ordinal) == true &&
+                                       (ex.Message.Contains("constraint", StringComparison.OrdinalIgnoreCase) ||
+                                        ex.Message.Contains("duplicate", StringComparison.OrdinalIgnoreCase) ||
+                                        ex.Message.Contains("unique", StringComparison.OrdinalIgnoreCase));
+
+            if (isConstraintViolation)
+            {
+                // Log constraint violations as warnings (expected in some concurrent scenarios)
+                logger.LogWarning(
+                    ex,
+                    "Database constraint violation: Attempted to insert or update data that violates a database constraint. " +
+                    "This may indicate a concurrency issue or a bug in the update logic. " +
+                    "Inner exception: {InnerExceptionType}",
+                    ex.InnerException?.GetType().Name ?? "unknown");
+            }
+            else
+            {
+                // Other DbUpdateExceptions are true errors
+                SaveChangesError(logger, ex);
+            }
+
+            throw;
+        }
         catch (Exception e)
         {
             SaveChangesError(logger, e);