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_findatender.go

@@ -3,7 +3,6 @@ package fetcher
 import (
 	"context"
 	"errors"
-	"fmt"
 	"log/slog"
 	"time"
 
@@ -16,13 +15,14 @@ import (
 // runFindAOpportunity polls Find a Tender every configured interval until ctx
 // is cancelled.
 func runFindAOpportunity(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	w := util.Worker{
+		Logger:   logger,
 		Name:     "FindAOpportunity",
 		Interval: conf.Sources.FindATender.Interval,
 		Task: func(ctx context.Context, since time.Time) error {
-			return fetchFindAOpportunity(ctx, conf, since)
+			return fetchFindAOpportunity(ctx, logger, conf, since)
 		},
 	}
 	return w.Run(ctx, since)
@@ -32,10 +32,10 @@ func runFindAOpportunity(
 // Tender. Finding nothing returns ErrNoNewReleases, so the worker still moves
 // its checkpoint on.
 func fetchFindAOpportunity(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	opportunities, err := util.AwaitTask(ctx, func() ([]*types.Opportunity, error) {
-		ft := findatender.New(conf, conf.Sources.FindATender.APIRoot)
+		ft := findatender.New(conf, conf.Sources.FindATender.APIRoot, findatender.WithLogger(logger))
 		defer ft.Shutdown()
 		return ft.With(ctx).FetchAll(since, true)
 	})
@@ -46,7 +46,7 @@ func fetchFindAOpportunity(
 		return err
 	}
 
-	slog.Info(fmt.Sprintf("Found %d opportunities", len(opportunities)))
+	logger.Info("fetched opportunities", "count", len(opportunities))
 
 	return nil
 }