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_test.go

@@ -0,0 +1,53 @@
+package monitor
+
+import (
+	"context"
+	"testing"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
+)
+
+// Maintenance cache resolves parent relationships correctly.
+func TestIsInMaintenance_UsesCache(t *testing.T) {
+	ms := newMockStore()
+	ms.maintenance[10] = true // direct maintenance on group
+	e := newTestEngine(ms)
+	group := models.Site{
+		SiteConfig: models.SiteConfig{ID: 10, Name: "group", Type: "group"},
+		SiteState:  models.SiteState{Status: "UP"},
+	}
+	child := models.Site{
+		SiteConfig: models.SiteConfig{ID: 20, Name: "child", Type: "http", ParentID: 10},
+		SiteState:  models.SiteState{Status: "UP"},
+	}
+	injectSite(e, group)
+	injectSite(e, child)
+	e.refreshMaintenanceCache(context.Background())
+
+	if !e.isInMaintenance(10) {
+		t.Error("group should be in maintenance (direct)")
+	}
+	if !e.isInMaintenance(20) {
+		t.Error("child should be in maintenance (parent)")
+	}
+	if e.isInMaintenance(99) {
+		t.Error("unknown monitor should not be in maintenance")
+	}
+}
+
+// Global maintenance (monitor_id=0) applies to all monitors.
+func TestIsInMaintenance_GlobalMaintenance(t *testing.T) {
+	ms := newMockStore()
+	ms.maintenance[0] = true
+	e := newTestEngine(ms)
+	site := models.Site{
+		SiteConfig: models.SiteConfig{ID: 1, Name: "test", Type: "http"},
+		SiteState:  models.SiteState{Status: "UP"},
+	}
+	injectSite(e, site)
+	e.refreshMaintenanceCache(context.Background())
+
+	if !e.isInMaintenance(1) {
+		t.Error("all monitors should be in maintenance during global window")
+	}
+}