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/internal/widget/widget.go

@@ -40,7 +40,9 @@ type Fetcher interface {
 
 var registry = map[string]Fetcher{}
 
-func register(f Fetcher) { registry[f.Meta().Type] = f }
+// Register adds a fetcher — used by the api package for DB-backed widgets
+// (monitor/domain/system) that can't live in this leaf package.
+func Register(f Fetcher) { registry[f.Meta().Type] = f }
 
 // Get resolves a config.type to its fetcher.
 func Get(t string) (Fetcher, bool) {