feat(agenda): unified timeline of deadlines + appointments across projects

t-paliad-030. Adds `/agenda` — a single page that merges every visible
deadline and appointment into a day-grouped timeline, the third overview
surface alongside Dashboard and the per-resource lists.

- AgendaService: merges paliad.deadlines + paliad.appointments, gated by
  the same team-membership predicate used everywhere else; personal
  appointments stay creator-only. Items are sorted by date and tagged
  with urgency (overdue / today / tomorrow / this_week / later) so the
  client can apply the traffic-light colours without re-deriving buckets.
- GET /api/agenda?from&to&types and GET /agenda with the same server-side
  hydration pattern as /dashboard (JSON payload spliced into the shell so
  the timeline paints on first frame).
- Frontend: agenda.tsx + client/agenda.ts render a day-grouped timeline
  with type/range chips; filters round-trip through the URL.
- Sidebar entry under "Übersicht"; DE+EN i18n across all new keys.

cmd/server/main.go

@@ -116,6 +116,7 @@ func main() {
 			ChecklistInst:  services.NewChecklistInstanceService(pool, projectSvc),
 			Mail:           mailSvc,
 			Invite:         inviteSvc,
+			Agenda:         services.NewAgendaService(pool, users),
 		}
 		log.Println("Phase B services initialised")