Retire the v2 API cache, bound candidate offers, deflake link leak tests

- Bump the service worker API cache to v3 so activation deletes the v2
  cache, whose recipe-candidate responses used the full page URL as
  candidate_id; test that activation deletes v1 and v2 and keeps v3.
- Give candidate offers a 30-minute lifetime, drop expired offers when
  new ones are stored, and clear a dinner's offers on every hand
  grounding (candidate, link or manual) and on archive. A candidate
  search still running when its dinner is cleared stores nothing, so a
  stale candidate_id gets the same 400 as an unknown one.
- Make the link leak assertions in TestDinnerResponsesHideTokenInLinkPath
  and TestCreateRegularNeverNamesFromLink check phrases a ULID cannot
  contain, so a dinner id holding "KEY" no longer fails them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dinners/export_test.go

@@ -14,6 +14,23 @@ func SetGroundTimeout(t *testing.T, d time.Duration) {
 	t.Cleanup(func() { groundTimeout = old })
 }
 
+// SetOfferLifetime sets how long candidates ground a dinner until t ends.
+// A test calling it must not run in parallel.
+func SetOfferLifetime(t *testing.T, d time.Duration) {
+	old := offerLifetime
+	offerLifetime = d
+	t.Cleanup(func() { offerLifetime = old })
+}
+
+// HasOffers reports whether the service holds candidates offered for
+// dinner id.
+func (s *Service) HasOffers(id string) bool {
+	s.offersMu.Lock()
+	defer s.offersMu.Unlock()
+	_, ok := s.offers[id]
+	return ok
+}
+
 // RunGroundJob runs the KindGroundDinner handler for p in the caller's
 // goroutine, as the job runner would, and returns the grounding status it
 // reports.