feat: external importers + uptime monitors (merge plan M1)

Two tracks from the Beszel/Domain-Locker merge plan in ROADMAP.md:

- Importers: POST /api/import/external sniffs Homepage services.yml,
  Homarr JSON, and Dashy conf.yml into sections/items; SettingsDialog
  import now accepts all four formats.
- Monitors (M1): monitors + heartbeats schema, pure checkers for
  http/tcp/ping/dns/keyword/json-query/push, scheduler on app ctx,
  CRUD + check-now + push ingest API, /monitors + /monitors/:id pages
  (wouter), monitor board widget, ServiceDialog monitor opt-in.

Authored By: TDvorak <[email protected]>

Generated with Devin (https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/cmd/dash/main.go

@@ -42,8 +42,11 @@ func main() {
 	}
 	defer func() { _ = sqlDB.Close() }()
 
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
 	iconsDir := filepath.Join(dataDir, "icons")
-	router := api.NewRouter(logger, sqlDB, iconsDir)
+	router := api.NewRouter(ctx, logger, sqlDB, iconsDir)
 	if err := web.Register(router); err != nil {
 		logger.Fatal("register embedded UI", zap.Error(err))
 	}
@@ -51,9 +54,6 @@ func main() {
 	addr := ":" + envOr("DASH_PORT", "8080")
 	srv := &http.Server{Addr: addr, Handler: router, ReadHeaderTimeout: 10 * time.Second}
 
-	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
-	defer stop()
-
 	go func() {
 		logger.Info("listening", zap.String("addr", addr), zap.String("data", dataDir))
 		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {