Implement event deletion per spec §10 with role-based rules:
- Recorder can delete own events if no dependents exist
- Admin can cascade delete (tombstone target + all dependents)
- All deletions create immutable tombstone records
New modules:
- events/dependencies.py: find events depending on a target via shared animals
- events/delete.py: delete_event() with projection reversal
DependentEventsError exception added for 409 Conflict responses.
E2E test #6 implemented per spec §21.6.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
