Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/util/worker.go

@@ -36,6 +36,9 @@ type Worker struct {
 	ShutdownTimeout time.Duration
 	// Task is the work to perform on each tick.
 	Task Task
+	// Logger is what the worker logs to; nil means slog.Default(), looked up
+	// when Run starts.
+	Logger *slog.Logger
 }
 
 // Run executes Task immediately, then once per Interval, until ctx is
@@ -55,7 +58,11 @@ func (w Worker) Run(ctx context.Context, since time.Time) error {
 		timeout = DefaultWorkerShutdownTimeout
 	}
 
-	logger := slog.With("source", w.Name)
+	logger := w.Logger
+	if logger == nil {
+		logger = slog.Default()
+	}
+	logger = logger.With("source", w.Name)
 	logger.Info("starting worker", "since", since, "interval", w.Interval)
 
 	// workCtx is NOT cancelled immediately when ctx is. It gives an in-flight
@@ -123,7 +130,7 @@ func (w Worker) execute(
 		logger.Warn("task cancelled", "elapsed", elapsed)
 		return since
 	default:
-		logger.Error("task failed", "err", err, "elapsed", elapsed)
+		logger.Error("task failed", "error", err, "elapsed", elapsed)
 		return since
 	}
 }