feat(web,latest): pause and resume one Site's Lane with a mandatory expiry (#147)

Two owner-gated POST routes write the durable poll_lanes pause stamp the
poller's top-of-pass gate already reads: /admin/lanes/{site}/pause validates
the duration against the fixed 1h/6h/24h allow-list and the Site against the
registry, and /admin/lanes/{site}/resume zeroes the stamp. Both cap the form
body like the API path, answer with the freshly rendered Lanes block, and
never command the poller — the pause is a fact about the Site, so it
survives a restart. The Lanes page's c-ctrl slot now carries the pausebar:
Resume while paused, the duration select plus Pause while running, with the
paused phrase read from the live poll_lanes stamp so a press renders as
paused with no pass having run. Tests cover the round trips, rejections,
body caps, the pause-before-refusal ordering, fresh-poller survival, resume
restoring the full queue, and acquisition being unaffected.

backend/internal/web/admin.go

@@ -47,6 +47,8 @@ func (h *Handler) adminRoutes() []adminRoute {
 		{"GET /admin/series", h.adminSeries},
 		{"GET /admin/series/{key}", h.adminSeriesDetail},
 		{"POST /admin/series/{key}/poll", h.adminSeriesPoll},
+		{"POST /admin/lanes/{site}/pause", h.adminLanePause},
+		{"POST /admin/lanes/{site}/resume", h.adminLaneResume},
 		{"GET /ui/admin/lanes", h.uiLanes},
 		{"POST /readers/{id}/revoke", h.revokeReaderSessions},
 		{"POST /readers/{id}/clear-marks", h.clearReaderMarks},