feat(16.2-01): compile, decode and load plugin field types in cabana

- compileFieldTypes builds a per-boot field type registry (alias, core and
  cross-plugin collisions, element prefix, assets, option keys) passed down
  the compile functions; settings and relation forms refuse plugin aliases
- fields.yaml accepts a plugin alias with the general keys plus the type's
  options (typeOptions, element, translatable in the schema); CheckOptions
  runs at boot; CompileFormWithTypes for plugin tests
- save decodes plugin values inside the transaction (FormFieldError is a
  422, other errors opaque 500) and answers meta.notices (FieldNotice) only
  when a type reported something; show and save serve values through Load
- GET /field-types lists the types with versioned asset URLs; OpenAPI,
  schema.d.ts, README and docs/backend/field-types.md updated

modules/cabana/admin_openapi.go

@@ -261,6 +261,20 @@ func AdminAccountAvatarRemove() {}
 // @Router /navigation [get]
 func AdminNavigation() {}
 
+// AdminFieldTypes documents GET /field-types.
+//
+// @Summary Plugin field types
+// @Description Every admin form field type a plugin registers (pact.HasFormFieldTypes), sorted by name, with the URLs of its scripts and stylesheets (each with a ?v= content hash). The admin loads every script once after sign-in and keeps every stylesheet enabled on all pages; a form field whose `element` is set renders that custom element. An empty list when no plugin registers a type. Any backend session may call it.
+// @Tags admin
+// @Produce json
+// @Security BackendBearer
+// @Success 200 {object} FieldTypesResponse
+// @Failure 401 {object} ErrorEnvelope
+// @Failure 403 {object} ErrorEnvelope
+// @Failure 404 {object} ErrorEnvelope
+// @Router /field-types [get]
+func AdminFieldTypes() {}
+
 // AdminSettingsList documents GET /settings.
 //
 // @Summary List admin settings
@@ -356,7 +370,7 @@ func AdminListSchema() {}
 // AdminFormSchema documents the form schema route.
 //
 // @Summary Admin form schema
-// @Description The form of a controller, localized. `preview` is present when config_form.yaml declares a preview block: the form then has a read-only preview screen, which shows the fields whose context allows preview, the record actions and, when preview.headerPartial is set, that partial as a status hint. A `type: password` field and every other field the controller lists as virtual is sent in a save body and never has a value in a record response. `preset` on a text or mltext field names the text or mltext field it follows on the create form (type slug or exact; an mltext pair follows per locale) until the administrator edits it. A `type: permissioneditor` field carries `permissionOptions`, the permissions the controller offers the requesting administrator; its value in a record response and in a save body is an object of permission code to integer (radio mode 1 or -1, checkbox mode 1). A `type: relation` field over a protected foreign key is `readOnly` unless its contract declares the key writable.
+// @Description The form of a controller, localized. `preview` is present when config_form.yaml declares a preview block: the form then has a read-only preview screen, which shows the fields whose context allows preview, the record actions and, when preview.headerPartial is set, that partial as a status hint. A `type: password` field and every other field the controller lists as virtual is sent in a save body and never has a value in a record response. `preset` on a text or mltext field names the text or mltext field it follows on the create form (type slug or exact; an mltext pair follows per locale) until the administrator edits it. A `type: permissioneditor` field carries `permissionOptions`, the permissions the controller offers the requesting administrator; its value in a record response and in a save body is an object of permission code to integer (radio mode 1 or -1, checkbox mode 1). A `type: relation` field over a protected foreign key is `readOnly` unless its contract declares the key writable. A field whose type a plugin registers carries `element` (the custom-element tag the admin renders), `translatable` (its value is an object of content locale to string) and `typeOptions` (the type's own fields.yaml keys).
 // @Tags admin
 // @Produce json
 // @Security BackendBearer
@@ -454,7 +468,7 @@ func AdminList() {}
 // AdminCreate documents the record create route.
 //
 // @Summary Create an admin record
-// @Description Relation fields are sent by field name with ids ({"genre": 3, "artists": [4, 9]}); the response carries the same shape plus meta.labels.
+// @Description Relation fields are sent by field name with ids ({"genre": 3, "artists": [4, 9]}); the response carries the same shape plus meta.labels. A plugin field type's value is decoded by its type inside the save; when a type reports what it changed, meta.notices lists the localized messages per field (and content locale), and the key is absent otherwise.
 // @Tags admin
 // @Accept json
 // @Produce json
@@ -643,6 +657,7 @@ func AdminShow() {}
 // AdminUpdate documents the record update route.
 //
 // @Summary Update an admin record
+// @Description A plugin field type's value is decoded by its type inside the save. When a type reports what it changed, meta.notices lists the localized messages per field (and content locale), and the key is absent otherwise.
 // @Tags admin
 // @Accept json
 // @Produce json