The finished_at field previously stored only a date, causing bag "finished"
timeline events to sort before same-day brews. Now stores a full datetime
and uses the actual close time for timeline ordering.
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
Add EntityCounts struct and StatsRepository::entity_counts() to query
per-entity row counts. Include them in CachedStats so the home page can
read counts from cache instead of issuing six list queries with LIMIT 1.
Derive Default on StatsView for the fallback case.
