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/sites.go

@@ -0,0 +1,158 @@
+package monitor
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
+)
+
+func (e *Engine) SetActive(active bool) {
+	e.activeMu.Lock()
+	defer e.activeMu.Unlock()
+	if e.isActive != active {
+		e.isActive = active
+		status := "RESUMED (Active)"
+		if !active {
+			status = "PAUSED (Passive)"
+		}
+		e.AddLog(fmt.Sprintf("Engine %s", status))
+	}
+}
+
+func (e *Engine) IsActive() bool {
+	e.activeMu.RLock()
+	defer e.activeMu.RUnlock()
+	return e.isActive
+}
+
+func (e *Engine) GetAllSites() []models.Site {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	sites := make([]models.Site, 0, len(e.liveState))
+	for _, s := range e.liveState {
+		sites = append(sites, s)
+	}
+	return sites
+}
+
+func (e *Engine) GetLiveState() map[int]models.Site {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	cp := make(map[int]models.Site, len(e.liveState))
+	for k, v := range e.liveState {
+		cp[k] = v
+	}
+	return cp
+}
+
+func (e *Engine) addToTokenIndex(site models.Site) {
+	if site.Type == "push" && site.Token != "" {
+		e.tokenIndex[site.Token] = site.ID
+	}
+}
+
+func (e *Engine) removeFromTokenIndex(id int) {
+	for token, sid := range e.tokenIndex {
+		if sid == id {
+			delete(e.tokenIndex, token)
+			return
+		}
+	}
+}
+
+func (e *Engine) UpdateSiteConfig(cfg models.SiteConfig) {
+	e.mu.Lock()
+	if existing, ok := e.liveState[cfg.ID]; ok {
+		e.removeFromTokenIndex(cfg.ID)
+		existing.SiteConfig = cfg
+		e.liveState[cfg.ID] = existing
+		e.addToTokenIndex(existing)
+	}
+	e.mu.Unlock()
+
+	e.signalRecheck(cfg.ID)
+}
+
+func (e *Engine) RemoveSite(id int) {
+	e.mu.Lock()
+	e.removeFromTokenIndex(id)
+	delete(e.liveState, id)
+	e.mu.Unlock()
+	e.removeHistory(id)
+
+	e.probeResultsMu.Lock()
+	delete(e.probeResults, id)
+	e.probeResultsMu.Unlock()
+
+	e.recheckMu.Lock()
+	delete(e.recheck, id)
+	e.recheckMu.Unlock()
+}
+
+func (e *Engine) ToggleSitePause(id int) bool {
+	var (
+		paused bool
+		name   string
+	)
+	_, ok := e.applyState(id, func(s *models.Site) {
+		s.Paused = !s.Paused
+		paused = s.Paused
+		name = s.Name
+	})
+	if !ok {
+		return false
+	}
+	if paused {
+		e.AddLog(fmt.Sprintf("Monitor '%s' paused", name))
+	} else {
+		e.AddLog(fmt.Sprintf("Monitor '%s' resumed", name))
+	}
+	return paused
+}
+
+// applyState atomically reads, mutates, and writes back the live entry for id.
+// The mutator runs under the engine write lock and receives a pointer to the
+// CURRENT live state, so concurrent config edits, pauses, and heartbeats are
+// never clobbered by a stale snapshot. The mutator must only touch runtime /
+// check-result fields — config fields (Name/URL/Type/Token/Interval/AlertID/…)
+// are owned by UpdateSiteConfig and must not be written here. Returns the
+// post-mutation copy and whether the site still exists.
+func (e *Engine) applyState(id int, mutate func(s *models.Site)) (models.Site, bool) {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	cur, ok := e.liveState[id]
+	if !ok {
+		return models.Site{}, false
+	}
+	mutate(&cur)
+	e.liveState[id] = cur
+	return cur, true
+}
+
+func (e *Engine) GetDisplayStatus(site models.Site) string {
+	if site.Paused {
+		return "PAUSED"
+	}
+	if e.isInMaintenance(site.ID) {
+		return "MAINT"
+	}
+	return string(site.Status)
+}
+
+func (e *Engine) GetStateChanges(siteID int, limit int) []models.StateChange {
+	changes, err := e.db.GetStateChanges(context.Background(), siteID, limit)
+	if err != nil {
+		return nil
+	}
+	return changes
+}
+
+func (e *Engine) GetStateChangesSince(siteID int, since time.Time) []models.StateChange {
+	changes, err := e.db.GetStateChangesSince(context.Background(), siteID, since)
+	if err != nil {
+		return nil
+	}
+	return changes
+}