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/sites.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"net/url"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -184,6 +185,9 @@ func comixLatestChapter(seriesURL, body string) (latestChapter, bool) {
 	return maxChapter(re, body)
 }
 
+// kaganeLatestChapter scans the kagane series API JSON that the browser read
+// fetched from inside the page; the match rides on the property name,
+// regardless of the surrounding JSON shape.
 func kaganeLatestChapter(_, body string) (latestChapter, bool) {
 	return maxChapter(kaganeChapterRe, body)
 }
@@ -419,7 +423,8 @@ var sites = map[string]site{
 
 // browserBackedSites is derived from the registry: the Sites whose pages are
 // read through the browser sidecar, which are also the ones granted the longer
-// cooldown.
+// cooldown. Sorted so callers that range it (the due query, the browser
+// fetcher's dispatch) see a stable order instead of map-iteration noise.
 func browserBackedSites() []string {
 	out := make([]string, 0, len(sites))
 	for name, s := range sites {
@@ -427,5 +432,6 @@ func browserBackedSites() []string {
 			out = append(out, name)
 		}
 	}
+	sort.Strings(out)
 	return out
 }