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/config/conf_source_sleeper.go

@@ -0,0 +1,32 @@
+package config
+
+import "reflect"
+
+const JSCKSourceSleeper string = "sleeper"
+
+type JobSearchConfigSourceSleeper struct {
+	JobSearchConfigSource
+	BlockShutdown bool `cfg:"block_shutdown"`
+}
+
+/*
+ * General section functions
+ * See related methods in section.go
+ */
+func (c JobSearchConfigSourceSleeper) Map() map[string]any { return mapOf(c) }
+
+func (c JobSearchConfigSourceSleeper) Flatten() map[string]any {
+	return flattenOf(JSCKSourceSleeper, c)
+}
+
+func (c JobSearchConfigSourceSleeper) TypeOf(key string) (reflect.Kind, error) {
+	return typeOf(c, key)
+}
+
+func (c JobSearchConfigSourceSleeper) ValueOf(key string) (any, error) {
+	return valueOf(c, key)
+}
+
+func (c *JobSearchConfigSourceSleeper) SetValue(key string, value any) error {
+	return setValue(c, key, value)
+}