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/internal/web/cover.go

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"bookmarkmanager/backend/internal/store"
 	"context"
 	"log"
 	"net/http"
@@ -21,17 +22,6 @@ type CoverFetcher interface {
 // so it is validated at the boundary rather than passed through.
 var coverIDRe = regexp.MustCompile(`^[0-9a-f-]{36}$`)
 
-// coverTypes is the set of content types the proxy will echo back. A response
-// header sourced from a third party is not repeated verbatim: anything outside
-// this set is treated as "not a cover".
-var coverTypes = map[string]bool{
-	"image/webp": true,
-	"image/jpeg": true,
-	"image/png":  true,
-	"image/avif": true,
-	"image/gif":  true,
-}
-
 // coverTimeout bounds one proxied cover. Shorter than the fetcher's own
 // challenge budget on purpose: a browser page is waiting on this, and a cover
 // that has not arrived by now is better left as a broken slot than as a request
@@ -75,7 +65,7 @@ func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if !coverTypes[contentType] {
+	if !store.IsKaganeCoverContentType(contentType) {
 		log.Printf("kagane cover %s: unexpected content type %q", id, contentType)
 		http.NotFound(w, r)
 		return