refactor: decompose monitor.go and sqlstore.go into focused files

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.

internal/monitor/maintenance.go

@@ -0,0 +1,70 @@
+package monitor
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+func (e *Engine) maintenancePruner(ctx context.Context) {
+	ticker := time.NewTicker(maintPruneInterval)
+	defer ticker.Stop()
+
+	e.pruneMaintenanceWindows(ctx)
+
+	for {
+		select {
+		case <-ticker.C:
+			e.pruneMaintenanceWindows(ctx)
+		case <-ctx.Done():
+			return
+		}
+	}
+}
+
+func (e *Engine) pruneMaintenanceWindows(ctx context.Context) {
+	pruned, err := e.db.PruneExpiredMaintenanceWindows(ctx, e.maintRetention)
+	if err != nil {
+		e.AddLog(fmt.Sprintf("Maintenance prune error: %v", err))
+		return
+	}
+	if pruned > 0 {
+		e.AddLog(fmt.Sprintf("Pruned %d expired maintenance window(s)", pruned))
+	}
+}
+
+func (e *Engine) isInMaintenance(monitorID int) bool {
+	e.maintCacheMu.RLock()
+	defer e.maintCacheMu.RUnlock()
+	return e.maintCache[monitorID]
+}
+
+func (e *Engine) refreshMaintenanceCache(ctx context.Context) {
+	windows, err := e.db.GetActiveMaintenanceWindows(ctx)
+	if err != nil {
+		return
+	}
+
+	directMaint := make(map[int]bool)
+	var globalMaint bool
+	for _, w := range windows {
+		if w.MonitorID == 0 {
+			globalMaint = true
+		} else {
+			directMaint[w.MonitorID] = true
+		}
+	}
+
+	resolved := make(map[int]bool)
+	e.mu.RLock()
+	for id, site := range e.liveState {
+		if globalMaint || directMaint[id] || (site.ParentID > 0 && directMaint[site.ParentID]) {
+			resolved[id] = true
+		}
+	}
+	e.mu.RUnlock()
+
+	e.maintCacheMu.Lock()
+	e.maintCache = resolved
+	e.maintCacheMu.Unlock()
+}