feat(12.2-02): add the fileupload field with deferred uploads committed on save

- type: fileupload compiles the D-08 keys and binds to the model's attach.Relation at boot
- X-Session-Key (cabana.SessionKeyHeader) carries the form session key; RecordInput.SessionKey
- GET and POST .../{id}/files/{field}: list with pending uploads, multipart upload into attach.Store
- the create and update save attaches the session's pending files in its transaction
- swagger2openapi folds formData parameters into a multipart requestBody
- admin OpenAPI, TS types, conformance cases, README and forms docs

modules/cabana/auth.go

@@ -28,6 +28,7 @@ const (
 	msgForbidden          = "Forbidden"
 	msgNotFound           = "Not found"
 	msgServerError        = "Server error"
+	msgPayloadTooLarge    = "Payload too large"
 )
 
 // BackendUsers loads activated backend principals. It never reads frontend users.