Added fetch service and a mock Find a Tender server

Fetching:
- Add `fetch service`, which runs a worker per enabled source until
  SIGINT/SIGTERM (internal/fetcher)
- Add util.Worker: runs a task on an interval with a since-checkpoint
  that only advances on success, and a shutdown timeout for in-flight
  work; plus AwaitTask/AwaitTaskE for calls that ignore cancellation
- Add fts_mock and sleeper sources
- Sleeper: sleeps periodically to emulate work being done. Can be
  instructed to block the shutdown signal or handle it cleanly

Mock FTS server (cmd/mock-fts, internal/mock/findATender):
- Serves GET /api/1.0/ocdsReleasePackages with FTS paging, query
  validation, UK local time handling and per-client rate limiting
- Generates fictional releases from an embedded catalog, moving
  procedures through planning, tender and award, and keeps them in a
  JSON Lines store that survives restarts
- Add logging.AccessLog for combined/common access logs

internal/fetcher/worker_sleeper.go

@@ -0,0 +1,50 @@
+package fetcher
+
+import (
+	"context"
+	"time"
+
+	"git.avsdev.uk/Go/job-search/internal/config"
+	"git.avsdev.uk/Go/job-search/internal/util"
+)
+
+// sleepDuration is how long the sleeper's simulated work takes.
+const sleepDuration = 60 * time.Second
+
+// Test source that just sleeps for 60 seconds to simulate doing work
+func runSleeper(
+	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+) error {
+	blockShutdown := conf.Sources.Sleeper.BlockShutdown
+	w := util.Worker{
+		Name:     "Sleeper",
+		Interval: conf.Sources.Sleeper.Interval,
+		Task: func(taskCtx context.Context, since time.Time) error {
+			if blockShutdown {
+				return sleep(taskCtx, true, sleepDuration)
+			} else {
+				return sleep(ctx, false, sleepDuration)
+			}
+		},
+	}
+	return w.Run(ctx, since)
+}
+
+func sleep(
+	ctx context.Context, blockShutdown bool, duration time.Duration,
+) error {
+	return util.AwaitTaskE(ctx, func() error {
+		if blockShutdown {
+			time.Sleep(duration)
+			return nil
+		}
+		timer := time.NewTimer(duration)
+		defer timer.Stop()
+		select {
+		case <-timer.C:
+			return nil
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	})
+}