feat(web,store,latest): forced poll — Check now stamps a Series for the next Lane pass (#146)

The control writes series.force_poll_at (column landed in migration 0014)
and never commands the poller: pending is derived as force_poll_at >
latest_checked_at and self-clears because the check stamp is written before
the fetch. The due query's forced flag overrides the rest cutoff, the
Sighting-deferral and finished-only clauses, jumps the queue, and wakes a
sleeping browser Lane; it never overrides an empty series_url, the Bookmarks
join, the refusal backoff, the sidecar-down skip or the Lane gap.

ADRs: 0013-commands-through-the-database.

backend/internal/web/admin.go

@@ -44,6 +44,7 @@ func (h *Handler) adminRoutes() []adminRoute {
 		{"GET /admin/readers", h.adminReaders},
 		{"GET /admin/series", h.adminSeries},
 		{"GET /admin/series/{key}", h.adminSeriesDetail},
+		{"POST /admin/series/{key}/poll", h.adminSeriesPoll},
 		{"GET /ui/admin/lanes", h.uiLanes},
 		{"POST /readers/{id}/revoke", h.revokeReaderSessions},
 		{"POST /readers/{id}/clear-marks", h.clearReaderMarks},