Thread context.Context from callers instead of creating
context.Background() at every call site. Engine stores its
lifecycle ctx for use by triggerAlert goroutines. TUI Model
carries ctx for store operations in Cmd closures. CLI commands
create a root ctx and thread it through openStore, seed
functions, and init methods.
Only two intentional context.Background() remain in non-root
positions: engine constructor default (overridden by Start)
and drainWrites (parent already cancelled at shutdown).
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
