ascade-destroy occurrences; support events without a fixed start time

- Event#occurrences is now dependent: :destroy, so destroying an Event
  (directly, or via Node's cascade) removes its Occurrence rows instead
  of orphaning them.
- Renamed generate_occurences -> generate_occurrences (typo fix);
  callback and method updated together.
- Occurrence.generate returns early when event.start_time is nil, so a
  chapter with no sourced time produces zero occurrences instead of a
  guessed one.
- seed_chapter no longer falls back to 19:00 when start_time is absent;
  base_time/end_time are nil in that case.

Database-level foreign key cascade on occurrences.event_id is still
pending.

1 files changed, 2 insertions, 2 deletions

diff --git a/db/seeds/chapters.rb b/db/seeds/chapters.rb
index 0b83987..ad3c9a0 100644
--- a/db/seeds/chapters.rb
+++ b/db/seeds/chapters.rb
@@ -65,13 +65,13 @@ def seed_chapter(parent_id:, slug:, tag:, title_de:, description_de:,
 
   # Create events
   events.each do |ev|
-    base_time = Time.parse("#{Date.today.year}-01-01 #{ev[:start_time] || '19:00'}:00")
+    base_time = ev[:start_time] ? Time.parse("#{Date.today.year}-01-01 #{ev[:start_time]}:00") : nil
     node.events.create!(
       title:      title_de,
       location:   ev[:location] || location,
       rrule:      ev[:rrule],
       start_time: base_time,
-      end_time:   base_time + (ev[:duration_hours] || 2).hours,
+      end_time:   base_time && base_time + (ev[:duration_hours] || 2).hours,
       tag_list:   ev[:tag_list] || 'open-day'
     )
   end