feat(events): schedule, recurrence and the calendar (Phase 4)

The four closed recurrence shapes computed in the definition's own IANA zone,
a fourteen-day materialisation horizon with projections beyond it, series as a
managed thing, and the admin calendar that replaces the plugin this feature
exists to replace. An event now happens on its own.

No schema change: Phase 1 built every column this needed.

- events/recurrence.js is the ONE place an occurrence is computed, so the
  runner's expansion and the calendar's forecast cannot disagree. No date
  library added — Node ships the tzdata one would vendor, behind Intl.
- The runner's materialise leg is now two halves: expand, then sweep. The
  window starts at `now - grace`, so an occurrence nobody could have seen is
  never invented retroactively; the horizon is what makes the missed sweep
  mean anything for a recurrence.
- Publishing is the schedule switch and archiving turns it off, and publishing
  re-pins every occurrence that has not started.
- A projection is never drawn over an instant a run occupies, so a cancelled
  occurrence does not reappear as a forecast.

54 new tests, incl. the DST fixture set the plan asked for and three new
statements proved against a real MariaDB. Suite 1768/1711/56 skipped/1 fail
(pre-existing CRLF). Walked end to end on the local review stack.

Docs: RunicGateway/docs#PENDING

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

client/src/routes/admin/AdminLayout.jsx

@@ -133,6 +133,10 @@ export const NAV = [
     title: 'Events',
     items: [
       { to: '/admin/events', label: 'Events', icon: IconCalendar, roles: ['admin', 'editor', 'moderator'] },
+      // Phase 4. The same staff gate as the list beside it: a calendar is a read,
+      // and the arcs it manages are authoring gated on the buttons rather than
+      // on the row.
+      { to: '/admin/events/calendar', label: 'Calendar', icon: IconCalendar, roles: ['admin', 'editor', 'moderator'] },
     ],
   },
   {
@@ -218,6 +222,7 @@ const TITLES = {
   '/admin/engagement/sends': 'Send Log',
   '/admin/engagement/retention': 'Retention',
   '/admin/events': 'Events',
+  '/admin/events/calendar': 'Event calendar',
   '/admin/events/new': 'New event',
 }