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/go.mod

@@ -4,8 +4,10 @@ go 1.27
 
 require (
 	github.com/gin-gonic/gin v1.12.0
+	github.com/goccy/go-yaml v1.19.2
 	github.com/pressly/goose/v3 v3.28.0
 	go.uber.org/zap v1.28.0
+	golang.org/x/net v0.58.0
 	modernc.org/sqlite v1.59.0
 )
 
@@ -21,7 +23,6 @@ require (
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.30.1 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
-	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
@@ -42,7 +43,6 @@ require (
 	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/crypto v0.55.0 // indirect
-	golang.org/x/net v0.58.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.41.0 // indirect