fix(monitor): serialize DB writes through a single drained writer

Every check spawned `go e.db.Save*(...)` with the error discarded: a
fire-and-forget goroutine per log line, check, state change, and alert
health update. SaveLog ran a full-table prune DELETE on every insert and
SaveCheck a COUNT + conditional prune on every check, so the hot path
amplified each write into several statements. Nothing tracked these
goroutines, so at shutdown they raced the store's Close() — writes to a
closing DB, silently swallowed.

Introduce a single writer goroutine that drains a buffered channel of
typed dbWrite values (log/check/state-change/alert-health). Writes are
enqueued non-blocking; a saturated queue drops and notes it in the
in-memory log rather than blocking the check loop. Write errors are now
logged instead of discarded. Retention moves off the hot path: SaveLog
and SaveCheck become plain INSERTs, and PruneLogs/PruneCheckHistory/
PruneStateChanges run on a 10-minute timer inside the writer (single
keep-newest-N-per-site pass via a window function). state_changes was
previously never pruned — now bounded.

Add Engine.Stop(): cancels the engine's context, then waits for the
writer to drain every buffered write before returning. main wires it in
before the deferred store Close() so no write races a closed DB.

SQLite gains busy_timeout=5000 and synchronous=NORMAL, applied via the
DSN so every pooled connection inherits them (a post-open PRAGMA only
touches one connection); WAL moves to the DSN too. :memory: test DBs are
left as-is.

Tests: writer drains on Stop, Stop is idempotent, and the prune queries
keep newest-N per site / N logs on real SQLite. Full suite green under
-race.

internal/store/store.go

@@ -39,11 +39,13 @@ type Store interface {
 	SaveCheck(siteID int, latencyNs int64, isUp bool) error
 	SaveCheckFromNode(siteID int, nodeID string, latencyNs int64, isUp bool) error
 	LoadAllHistory(limit int) (map[int][]models.CheckRecord, error)
+	PruneCheckHistory() error
 
 	// State Changes
 	SaveStateChange(siteID int, fromStatus, toStatus, errorReason string) error
 	GetStateChanges(siteID int, limit int) ([]models.StateChange, error)
 	GetStateChangesSince(siteID int, since time.Time) ([]models.StateChange, error)
+	PruneStateChanges() error
 
 	// Nodes
 	RegisterNode(node models.ProbeNode) error
@@ -59,6 +61,7 @@ type Store interface {
 	// Logs
 	SaveLog(message string) error
 	LoadLogs(limit int) ([]string, error)
+	PruneLogs() error
 
 	// Maintenance Windows
 	GetActiveMaintenanceWindows() ([]models.MaintenanceWindow, error)