Prefetch Kagane covers during latest polling (#51)

## Summary
- Add an optional browser-backed cover fetcher to the latest-chapter poller.
- Prefetch missing Kagane covers during the existing due-series cycle and persist them before a Reader opens the web UI.
- Keep chapter polling, cooldown stamping, and on-first-view fallback independent from cover failures.

## Behavior and safety
- Stored Kagane covers are detected before browser work, so later poll cycles do not refetch them.
- Nil cover fetchers and non-Kagane series retain the existing behavior.
- Shared Kagane image-id and content-type validation prevents challenge or non-image responses from poisoning persistent cover storage.
- The browser is wired into both the chapter and cover poller paths from the composition root.

## Verification
- `go test ./...`
- Focused latest, store, and web package tests
- Deterministic tests cover missing covers, cached covers, failed fetches, invalid content types, nil fetchers, and non-Kagane series.

Closes #45

Reviewed-on: #51
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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,