alert: construct + run engine; expose hub to handlers

- Construct notification.NewHub and alert.NewEngine at boot in cmd/server/main.go
- Start go alertEngine.Run(ctx) after construction, before the HTTP listener
- Wire AlertEngine and NotificationHub into rmhttp.Deps (fields already existed)
- Remove the TODO(G1) in the offline sweeper; now calls NotifyHostOffline per ID

cmd/server/main.go

@@ -12,8 +12,10 @@ import (
 	"syscall"
 	"time"
 
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/alert"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/auth"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/crypto"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/notification"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/config"
 	rmhttp "gitea.dcglab.co.uk/steve/restic-manager/internal/server/http"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/maintenance"
@@ -82,19 +84,24 @@ func run() error {
 	hub := ws.NewHub()
 	jobHub := ws.NewJobHub()
 
+	notifHub := notification.NewHub(st, aead, cfg.BaseURL)
+	alertEngine := alert.NewEngine(st, notifHub)
+
 	renderer, err := ui.New()
 	if err != nil {
 		return fmt.Errorf("ui: %w", err)
 	}
 
 	deps := rmhttp.Deps{
-		Cfg:     cfg,
-		Store:   st,
-		AEAD:    aead,
-		Hub:     hub,
-		JobHub:  jobHub,
-		UI:      renderer,
-		Version: version,
+		Cfg:             cfg,
+		Store:           st,
+		AEAD:            aead,
+		Hub:             hub,
+		JobHub:          jobHub,
+		AlertEngine:     alertEngine,
+		NotificationHub: notifHub,
+		UI:              renderer,
+		Version:         version,
 	}
 
 	// First-run bootstrap: if the users table is empty, mint a one-time
@@ -126,6 +133,8 @@ func run() error {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
+	go alertEngine.Run(ctx)
+
 	errCh := make(chan error, 1)
 	go func() {
 		slog.Info("server listening", "addr", cfg.Listen, "version", version)
@@ -177,9 +186,9 @@ func run() error {
 				cutoff := time.Now().Add(-90 * time.Second)
 				if ids, err := st.MarkHostsOfflineStaleReturnIDs(ctx, cutoff); err == nil && len(ids) > 0 {
 					slog.Info("marked hosts offline (stale heartbeat)", "n", len(ids))
-					// TODO(G1): notify engine once deps.AlertEngine is wired.
-					// for _, id := range ids { alertEngine.NotifyHostOffline(id) }
-					_ = ids
+					for _, id := range ids {
+						alertEngine.NotifyHostOffline(id)
+					}
 				}
 			case <-pendingDrainTick.C:
 				srv.DrainAllDue(ctx)