Add RRULE humanizer and wire events into nodes#showerdgeist-revive-events

- app/models/concerns/rrule_humanizer.rb: new concern included into
  Event, renders recurring schedule as natural-language German or
  English from RRULE string; handles WEEKLY/MONTHLY, biweekly
  (INTERVAL=2), ordinal weekday positions (1TU, -1TH, -2WE),
  BYMONTH single-month exclusions (December pause convention);
  gracefully returns nil for COUNT/UNTIL/unrecognized shapes
- test/models/concerns/rrule_humanizer_test.rb: 15 tests covering
  all distinct RRULE shapes found in production data
- app/helpers/nodes_helper.rb: add event_schedule_text helper
  combining humanize_rrule with start_time formatting
- app/views/nodes/show.html.erb: add events row, conditionally
  rendered when node has associated events
- config/locales/de.yml, en.yml: add event_schedule_time,
  event_schedule_unrecognized, event_schedule_none keys

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