checkGroup() now detects status transitions and persists them as
state change records, same as individual monitors. Groups also
get StatusChangedAt updates and log entries for down/recovery
events. This enables history and SLA views for group monitors.
Split monitor.go (1131→350 lines) into alerts.go, checks.go,
sites.go, and maintenance.go by concern. Split sqlstore.go
(833→470 lines) into sqlstore_alerts.go, sqlstore_history.go,
and sqlstore_maintenance.go by domain. Tests move with their
implementation. Pure reorganization — no behavioral changes.
lerko
