fix: address final-review findings on the latest-chapter poller

Wires the five poll env vars into docker-compose (the documented kill
switch was inert), skips fetching unknown sites and non-https URLs
before spending a request, and makes runOnce's summary log fire on
empty and cancelled ticks. Records the accepted non-atomic Get+Upsert
window and the one-interval startup delay in the docs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/latest_sites.go

@@ -33,9 +33,12 @@ var demonicChapterRe = regexp.MustCompile(`chaptered\.php\?manga=\d+&(?:amp;)?ch
 // The userscript's asura rule additionally requires the anchor text to match
 // /Chapter\s+[\d.]+/i. That check exists only to skip the "First Chapter"
 // shortcut, which points at chapter/1 and therefore can never win a maximum, so
-// it is redundant here. Scoping the pattern to this series' own slug replaces it
-// with a stronger guarantee: a chapter link belonging to some other series
-// cannot contribute even if the page starts carrying them.
+// it is redundant here. For asura, scoping the pattern to this series' own slug
+// replaces it with a stronger guarantee: a chapter link belonging to some other
+// series cannot contribute even if the page starts carrying them. demonic has no
+// such guarantee — demonicChapterRe matches any chaptered.php?manga=<id> anchor
+// with no per-series scoping, because the stored series_id for demonic is a
+// slug, not the numeric id the URL carries, so it cannot easily be scoped.
 func latestChapterFrom(site, seriesURL, body string) (latestChapter, bool) {
 	var re *regexp.Regexp
 	switch site {