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/alert/alert_test.go

@@ -0,0 +1,109 @@
+package alert
+
+import (
+	"encoding/json"
+	"go-upkeep/internal/models"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestHTTPProviderDiscord(t *testing.T) {
+	var received map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&received)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	p := GetProvider(models.AlertConfig{Type: "discord", Settings: map[string]string{"url": srv.URL}})
+	if err := p.Send("Test Title", "Test Body"); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+
+	if received["content"] != "**Test Title**\nTest Body" {
+		t.Errorf("unexpected payload: %s", received["content"])
+	}
+}
+
+func TestHTTPProviderSlack(t *testing.T) {
+	var received map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&received)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	p := GetProvider(models.AlertConfig{Type: "slack", Settings: map[string]string{"url": srv.URL}})
+	if err := p.Send("Alert", "Message"); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+
+	if received["text"] != "*Alert*\nMessage" {
+		t.Errorf("unexpected payload: %s", received["text"])
+	}
+}
+
+func TestHTTPProviderWebhook(t *testing.T) {
+	var received map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&received)
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	p := GetProvider(models.AlertConfig{Type: "webhook", Settings: map[string]string{"url": srv.URL}})
+	if err := p.Send("Title", "Body"); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+
+	if received["title"] != "Title" || received["message"] != "Body" || received["status"] != "alert" {
+		t.Errorf("unexpected webhook payload: %v", received)
+	}
+}
+
+func TestHTTPProviderErrorOnHTTP4xx(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(403)
+	}))
+	defer srv.Close()
+
+	p := GetProvider(models.AlertConfig{Type: "discord", Settings: map[string]string{"url": srv.URL}})
+	if err := p.Send("Test", "Test"); err == nil {
+		t.Fatal("expected error on 403 response")
+	}
+}
+
+func TestNtfyProvider(t *testing.T) {
+	var title, body string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		title = r.Header.Get("Title")
+		buf := make([]byte, 1024)
+		n, _ := r.Body.Read(buf)
+		body = string(buf[:n])
+		w.WriteHeader(200)
+	}))
+	defer srv.Close()
+
+	p := GetProvider(models.AlertConfig{Type: "ntfy", Settings: map[string]string{
+		"url":   srv.URL,
+		"topic": "test",
+	}})
+	if err := p.Send("Alert Title", "Alert Body"); err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+
+	if title != "Alert Title" {
+		t.Errorf("expected title 'Alert Title', got '%s'", title)
+	}
+	if body != "Alert Body" {
+		t.Errorf("expected body 'Alert Body', got '%s'", body)
+	}
+}
+
+func TestGetProviderUnknown(t *testing.T) {
+	p := GetProvider(models.AlertConfig{Type: "unknown"})
+	if p != nil {
+		t.Error("expected nil for unknown provider type")
+	}
+}