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/fetcher/worker_sleeper.go

@@ -2,6 +2,7 @@ package fetcher
 
 import (
 	"context"
+	"log/slog"
 	"time"
 
 	"git.avsdev.uk/Go/job-search/internal/config"
@@ -14,10 +15,11 @@ const sleepDuration = 60 * time.Second
 // runSleeper runs the sleeper test source until ctx is cancelled: every
 // configured interval it simulates work by sleeping for sleepDuration.
 func runSleeper(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	blockShutdown := conf.Sources.Sleeper.BlockShutdown
 	w := util.Worker{
+		Logger:   logger,
 		Name:     "Sleeper",
 		Interval: conf.Sources.Sleeper.Interval,
 		Task: func(taskCtx context.Context, since time.Time) error {