feat(latest): prefetch kagane covers during polling

Add an optional cover fetcher to the latest-chapter poller and wire the shared browser into it. Each due Kagane series now checks persistent storage, fetches a missing cover once, and keeps chapter polling and cooldown bookkeeping independent when cover work fails.\n\nCentralize Kagane image-id and content-type validation so prefetched bytes cannot poison the on-first-view fallback, and cover the missing, cached, failure, nil-fetcher, invalid-type, and non-Kagane paths with deterministic poll-cycle tests.\n\nRefs #45

backend/main.go

@@ -335,10 +335,15 @@ func main() {
 
 // newLatestPoller wires the configured cooldowns and fetchers into the poller.
 func newLatestPoller(s *store.Store, cfg LatestPoll, fetch, browser latest.Fetcher) *latest.Poller {
+	var covers latest.CoverFetcher
+	if f, ok := browser.(latest.CoverFetcher); ok {
+		covers = f
+	}
 	return &latest.Poller{
 		Store:           s,
 		Fetch:           fetch,
 		BrowserFetch:    browser,
+		CoverFetch:      covers,
 		Now:             time.Now,
 		Cooldown:        cfg.Cooldown,
 		BrowserCooldown: cfg.BrowserCooldown,