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/adguard.go

@@ -11,7 +11,7 @@ import (
 // AdGuard Home REST API: GET /control/stats with HTTP basic auth.
 type adguard struct{}
 
-func init() { register(adguard{}) }
+func init() { Register(adguard{}) }
 
 func (adguard) Meta() Type {
 	return Type{

apps/backend/internal/widget/clock.go

@@ -10,7 +10,7 @@ import (
 // it and the add-widget dialog can offer it like any other type.
 type clock struct{}
 
-func init() { register(clock{}) }
+func init() { Register(clock{}) }
 
 func (clock) Meta() Type {
 	return Type{

apps/backend/internal/widget/immich.go

@@ -10,7 +10,7 @@ import (
 // Immich REST API: GET /api/statistics with an x-api-key header.
 type immich struct{}
 
-func init() { register(immich{}) }
+func init() { Register(immich{}) }
 
 func (immich) Meta() Type {
 	return Type{

apps/backend/internal/widget/pihole.go

@@ -11,7 +11,7 @@ import (
 // jarvis: v5 only for now — v6 (/api/...) lands when someone runs it.
 type pihole struct{}
 
-func init() { register(pihole{}) }
+func init() { Register(pihole{}) }
 
 func (pihole) Meta() Type {
 	return Type{

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) {