feat(17-01): richeditor field type on the seaglass sanitizer

- seaglass (new module): bluemonday v1.0.27 policy ported from Journal
  without the Blog Editor rules, byte-stable token comparison,
  SanitizeReport, Sanitize, Check, Removals
- cabana: type richeditor (common keys plus size); every create and update
  value sanitized before Fill, non-string values are a 422, removals become
  meta.notices; settings and relation forms refuse the type
- backend lang richeditor.removed_* and markup_changed (en, pl plurals)
- TestSeaglassSmoke, TestCompanionsRichEditorSmoke, README and docs

go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/knadh/koanf/providers/env/v2 v2.0.1
 	github.com/knadh/koanf/providers/file v1.2.1
 	github.com/knadh/koanf/v2 v2.3.6
+	github.com/microcosm-cc/bluemonday v1.0.27
 	github.com/nicksnyder/go-i18n/v2 v2.6.1
 	github.com/riverqueue/river v0.47.0
 	github.com/riverqueue/river/riverdriver/riverdatabasesql v0.47.0
@@ -42,6 +43,7 @@ require (
 	dario.cat/mergo v1.0.2 // indirect
 	github.com/Azure/go-ansiterm v0.0.0-20250102033503-faa5f7b0171c // indirect
 	github.com/Microsoft/go-winio v0.6.2 // indirect
+	github.com/aymerick/douceur v0.2.0 // indirect
 	github.com/cenkalti/backoff/v4 v4.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/containerd/errdefs v1.0.0 // indirect
@@ -64,6 +66,7 @@ require (
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/googleapis/gax-go/v2 v2.19.0 // indirect
+	github.com/gorilla/css v1.0.1 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect