.NET structured logging with ILogger and source-generated LoggerMessage

// ======================================================================
// Q: How do you log properly in .NET?
//
// ILogger<T> via DI, log levels per namespace in configuration, and never
// passwords, tokens or personal data in logs. Export to Application Insights /
// OpenTelemetry (Q33).
// ======================================================================

// ✓ structured logging: OrderId / CustomerId become searchable fields
logger.LogInformation("Order {OrderId} placed by {CustomerId}", order.Id, customerId);

// ✗ interpolation: the fields are lost and the string is built even if the level is off
logger.LogInformation($"Order {order.Id} placed by {customerId}");

// For hot paths: source-generated logging, without allocations
[LoggerMessage(Level = LogLevel.Warning, Message = "ERP rejected {OrderNumber}: {Reason}")]
static partial void LogErpRejected(ILogger logger, string orderNumber, string reason);