[feature] content system — CMS admin + API integration

[baldrys-ed]

• ContentType + ContentEntry entities with soft-delete, timestamps, sort
• ContentTypeRepository + ContentEntryRepository (name-based lookups)
• ContentTypeController (index/create/update) with schema builder
• SupportsContentEntriesOperation trait — opt-in entries CRUD for any CrudController
• DynamicEntryType — PRE_SET_DATA-driven form for arbitrary schema (text/textarea/wysiwyg/image/file/collection)
• SchemaFieldType / SchemaItemFieldType — schema builder UI
• CollectionEntrySortTypeTrait — sort order field for nested collections
• ContentEntryUploadSubscriber — file URI→path on load; sort collections on persist
• ContentEntriesViewSubscriber — populates HasContentEntriesInterface views
• HasContentEntriesInterface + HasContentEntriesTrait — API view integration
• ContentEntryView — normalises entry data (casts sortOrder to int)
• ContentExtension / ContentRuntime — Twig helpers: content(), entriesFor()
• Twig templates: content/type/, content/entries/, content/blocks/_generic
• entries option in CrudControllerConfigurator + list.html.twig entries button
• Instantiator: full inheritance chain traversal
• collection/index.js: _prototypeKey isolation; schemaField/index.js toggle
• horizontal.html.twig: compound+label=false fix; layout.html.twig: data-prototype-key
• services.php: ContentTypeController registration + Entity/Api/View excludes
• Translations: content+intl-icu.en.yaml; cms+intl-icu.en.yaml common.content_entries
• CONTENT_SYSTEM.md: usage guide (entities, repos, CMS integration, Twig, API)