Rewrite chapter seeds: multi-event support, drop is_primary, fix data

- seed_chapter now accepts events: array instead of flat rrule:/
  start_time: params; each event hash supports rrule:, start_time:,
  tag_list:, location:, duration_hours:
- Chapters with multiple open days now represented correctly
  (Stralsund: Thursday Chaostreff + 2nd/4th Saturday OpenSpace;
  Hamburg: 2nd Friday + last Tuesday; Stuttgart: 1st Tuesday +
  3rd Wednesday; Freiburg: Mon+Tue open + biweekly Plenum;
  Backnang: 3rd Sunday + 1st Tuesday; Tübingen: last Sunday +
  2nd Monday at different venues)
- is_primary removed from migration entirely — replaced by
  tag_list: 'open-day' on events
- Stale EN descriptions corrected: Berlin, Darmstadt, Erlangen,
  Essen, Freiburg, Göttingen, Hamburg, Hannover, Karlsruhe,
  Paderborn, Stuttgart, Ulm
- chaostreff-stralsund duplicate entry removed (Port39 is erfa only)

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