fix: keep the legacy cover heal and no-chapter byte count (#94)

Restore two review findings against spec claims 'the Poll keeps its own
Cover policy' and 'pinning is the only behavioural change': prefetchCover
now also runs on ticks where the page fetch itself fails (the heal is
independent of the page read, and its source may answer while the origin
does not), and the no-chapter log surfaces the fetched body length again
via a BodyLen fact on seriesRead. Browser dispatch iterates the sorted
browser site list so its outcome cannot depend on map order.

backend/internal/latest/browser.go

@@ -93,10 +93,11 @@ func (f *BrowserFetcher) Close() {
 // which is what the entry's LatestChapter parse expects.
 func (f *BrowserFetcher) Get(ctx context.Context, seriesURL string) (string, int, error) {
 	var body string
-	for _, s := range sites {
-		if s.Browser == nil {
-			continue
-		}
+	// Sorted order (browserBackedSites sorts) makes dispatch deterministic:
+	// entries' Read funcs are expected to refuse any address owned by another
+	// Site, and the loop must not depend on that staying true.
+	for _, name := range browserBackedSites() {
+		s := sites[name]
 		read, ok := s.Browser.Read(seriesURL, &body)
 		if !ok {
 			continue