refactor(monitor): encapsulate engine state, add graceful shutdown and tests

Replace all monitor package-level mutable state with Engine struct.
All state (liveState, logStore, histories, tokenIndex, HTTP clients)
is now encapsulated in Engine, created via NewEngine(store).

Key changes:
- Engine struct holds all monitor state with proper mutex protection
- Engine.Start(ctx) and monitorRoutine respect context cancellation
  for graceful shutdown — no more leaked goroutines
- cluster.runFollowerLoop also respects context for clean exit
- Token index (map[string]int) for O(1) push heartbeat lookup,
  replacing O(n) linear scan through LiveState
- UpdateSiteConfig preserves 8 runtime fields instead of copying
  17 config fields individually
- triggerAlert goroutines get 30s timeout context
- All consumers (TUI, server, cluster, main) receive *Engine via
  constructor/parameter — no package-level state access
- main.go creates context.WithCancel, passes to engine and cluster

First test suite: 12 tests across store and alert packages
- Store: CRUD for sites/alerts/users, push token generation,
  import/export round-trip, check history persistence
- Alert: Discord/Slack/Webhook payload format, HTTP 4xx error
  propagation, Ntfy headers, unknown provider returns nil

internal/monitor/history.go

@@ -1,10 +1,6 @@
 package monitor
 
-import (
-	"go-upkeep/internal/store"
-	"sync"
-	"time"
-)
+import "time"
 
 const maxHistoryLen = 30
 
@@ -15,19 +11,14 @@ type SiteHistory struct {
 	UpChecks    int
 }
 
-var (
-	histories = make(map[int]*SiteHistory)
-	historyMu sync.RWMutex
-)
-
-func InitHistoryFromStore(s store.Store) {
-	all, err := s.LoadAllHistory(maxHistoryLen)
+func (e *Engine) InitHistory() {
+	all, err := e.db.LoadAllHistory(maxHistoryLen)
 	if err != nil {
-		AddLog("Failed to load check history: " + err.Error())
+		e.AddLog("Failed to load check history: " + err.Error())
 		return
 	}
-	historyMu.Lock()
-	defer historyMu.Unlock()
+	e.histMu.Lock()
+	defer e.histMu.Unlock()
 	for siteID, records := range all {
 		h := &SiteHistory{}
 		for _, r := range records {
@@ -38,21 +29,21 @@ func InitHistoryFromStore(s store.Store) {
 			h.Latencies = append(h.Latencies, time.Duration(r.LatencyNs))
 			h.Statuses = append(h.Statuses, r.IsUp)
 		}
-		histories[siteID] = h
+		e.histories[siteID] = h
 	}
 	if len(all) > 0 {
-		AddLog("Loaded check history from database")
+		e.AddLog("Loaded check history from database")
 	}
 }
 
-func RecordCheck(siteID int, latency time.Duration, isUp bool) {
-	historyMu.Lock()
-	defer historyMu.Unlock()
+func (e *Engine) recordCheck(siteID int, latency time.Duration, isUp bool) {
+	e.histMu.Lock()
+	defer e.histMu.Unlock()
 
-	h, ok := histories[siteID]
+	h, ok := e.histories[siteID]
 	if !ok {
 		h = &SiteHistory{}
-		histories[siteID] = h
+		e.histories[siteID] = h
 	}
 
 	h.TotalChecks++
@@ -70,15 +61,13 @@ func RecordCheck(siteID int, latency time.Duration, isUp bool) {
 		h.Statuses = h.Statuses[len(h.Statuses)-maxHistoryLen:]
 	}
 
-	if db != nil {
-		go func() { _ = db.SaveCheck(siteID, latency.Nanoseconds(), isUp) }()
-	}
+	go func() { _ = e.db.SaveCheck(siteID, latency.Nanoseconds(), isUp) }()
 }
 
-func GetHistory(siteID int) (SiteHistory, bool) {
-	historyMu.RLock()
-	defer historyMu.RUnlock()
-	h, ok := histories[siteID]
+func (e *Engine) GetHistory(siteID int) (SiteHistory, bool) {
+	e.histMu.RLock()
+	defer e.histMu.RUnlock()
+	h, ok := e.histories[siteID]
 	if !ok {
 		return SiteHistory{}, false
 	}
@@ -93,8 +82,8 @@ func GetHistory(siteID int) (SiteHistory, bool) {
 	return cp, true
 }
 
-func RemoveHistory(siteID int) {
-	historyMu.Lock()
-	defer historyMu.Unlock()
-	delete(histories, siteID)
+func (e *Engine) removeHistory(siteID int) {
+	e.histMu.Lock()
+	defer e.histMu.Unlock()
+	delete(e.histories, siteID)
 }