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/fetcher.go

@@ -0,0 +1,45 @@
+package fetcher
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"sync"
+	"time"
+
+	"git.avsdev.uk/Go/job-search/internal/config"
+	"git.avsdev.uk/Go/job-search/internal/util"
+)
+
+var ErrNoNewReleases = fmt.Errorf("%w: no new releases", util.ErrWorkerNoWork)
+
+func Run(
+	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+) {
+	var wg sync.WaitGroup
+
+	start := func(
+		name string,
+		run func(context.Context, *config.JobSearchConfig, time.Time) error,
+	) {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if err := run(ctx, conf, since); err != nil {
+				slog.Error("source stopped", "source", name, "err", err)
+			}
+		}()
+	}
+
+	if conf.Sources.FindATender.Enabled {
+		start("FindATender", runFindAOpportunity)
+	}
+	if conf.Sources.FTSMock.Enabled {
+		start("FTSMock", runFTSMock)
+	}
+	if conf.Sources.Sleeper.Enabled {
+		start("Sleeper", runSleeper)
+	}
+
+	wg.Wait()
+}