Add background timeline rebuild task (mirroring stats cache pattern)
that refreshes denormalized timeline event snapshots when entities are
updated. Includes cascade logic so editing a roaster refreshes timeline
events for its roasts, bags, brews, and cups.
- Add update_by_entity/delete_by_entity/delete_all to TimelineEventRepository
- Add TimelineInvalidator with debounced background rebuild task
- Add invalidate() calls to all 7 entity update handlers
- Add delete_by_entity cleanup to define_delete_handler! macro
- Add gear_id filter to BrewFilter for cascade traversal
- Add `brewlog timeline rebuild` CLI command for full rebuild
- Add 5 integration tests for timeline sync behavior
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