fix(web): statusline tokens, unreachable dot, reachability test spans (#145)

Second review round (spec + standards):
- Polling-off and unreachable render as statusline tokens; the unreachable
  span drops mark-strong so the patina dot never sits next to red text.
- Reachability tests assert on the rendered span, which 'reachable' and
  'unreachable' substrings never could.
- A pass-log query failure now reads as reachable (no evidence rule) instead
  of condemning the browser, and admin.go loses its dead time import.
- startLatestPoller's doc no longer claims the admin page reads the poller;
  AGENTS.md carries the RefuseBackoff rename.

backend/main.go

@@ -343,7 +343,8 @@ func newLatestPoller(s *store.Store, cfg LatestPoll, fetch, browser latest.Fetch
 // HTTP client cannot be built. Any problem here is logged and skipped: this
 // feature going missing degrades the service to userscript-only latest-chapter
 // tracking, which is exactly how it behaved before. It returns the running
-// Poller, or nil when there is none — the admin page's Lane status reads it.
+// Poller, or nil when there is none; the caller starts it for its own sake —
+// the Lanes page reads the pass log, so no return value is wired anywhere.
 func startLatestPoller(ctx context.Context, s *store.Store, cfg LatestPoll, browser latest.Fetcher) *latest.Poller {
 	if !cfg.Enabled {
 		log.Println("latest-chapter poller: disabled by config")