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/mock/findATender/doc.go

@@ -0,0 +1,13 @@
+// Package mock implements a mock of the Find a Tender (FTS) OCDS release
+// package endpoint (GET /api/1.0/ocdsReleasePackages): a catalog of buyers,
+// suppliers, subjects and methods loaded from catalog.json, a persistent
+// release store, a generator with a background worker, the HTTP handler and a
+// per-client rate limiter.
+package mock
+
+// Files kept in the mock-data directory.
+//
+//	ReleasesFile is created and appended to by the mock.
+const (
+	ReleasesFile = "releases.jsonl"
+)