Series URL repair: owner-typed, gated by the poller's own fetch gate (#151)

backend/internal/latest/poller.go

@@ -710,7 +710,7 @@ func (p *Poller) waitCovers() {
 	p.coverWG.Wait()
 }
 
-// fetchableSeriesURL reports whether site is a Site the registry knows and
+// FetchableSeriesURL reports whether site is a Site the registry knows and
 // seriesURL is safe to hand to a fetcher: an https URL whose host matches the
 // Site's pinned hostname exactly. series_url comes from client-supplied PUT
 // bodies, so this is a defence against the poller being used to probe
@@ -719,7 +719,11 @@ func (p *Poller) waitCovers() {
 // browser Site guards a control that executes JavaScript and carries cookies,
 // a parser Site guards a wasted request — but the rule is one rule, from the
 // registry.
-func fetchableSeriesURL(site, seriesURL string) bool {
+//
+// The owner's series URL repair (issue #151) is a second caller: the web
+// layer validates with this same gate before storing a repair, so there is
+// never a second copy of it.
+func FetchableSeriesURL(site, seriesURL string) bool {
 	s, known := sites[site]
 	if !known {
 		return false