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.

0 files changed, 0 insertions, 0 deletions