fix: registration fills only an explicit allow-list and never takes the IP columns from the body

User.RegisterFillable lists what a registration body may set: name,
surname, email, password, preferred_locale and marketing_consent.
RegisterUser fills from it instead of Fillable, so organisation_id,
organisation_role, is_onboarded and must_change_password can no longer be
mass-assigned through POST /register, and created_ip_address and
last_ip_address come only from RegisterOptions.IP. Fillable is unchanged
for the admin form and trusted application code.

models/user.go

@@ -81,8 +81,12 @@ func (User) AttachRelations() []attach.Relation {
 	return []attach.Relation{{Name: AvatarField, Public: true}}
 }
 
-// Fillable is the register/update field list. Username, PIN, and GDPR columns
-// stay out of Phase 7 because no ported handler reads them.
+// Fillable is the mass-assignment backstop of the admin Users form and of
+// trusted application code: cabana intersects it with the form's writable
+// fields (and an application's form extensions) and keeps organisation_id
+// protected. Registration does not use it: RegisterUser fills only
+// RegisterFillable. Username, PIN and GDPR columns stay out because no
+// ported handler reads them.
 func (User) Fillable() []string {
 	return []string{
 		"name", "surname", "email", "password", "created_ip_address", "last_ip_address",
@@ -91,6 +95,20 @@ func (User) Fillable() []string {
 	}
 }
 
+// RegisterFillable is the explicit allow-list of the columns a registration
+// body may set (POST /register and controllers.RegisterUser): name, surname,
+// email, password (stored as its hash), preferred_locale and
+// marketing_consent. A registration body can never set organisation_id or
+// organisation_role (membership is the administrator's or the
+// application's), created_ip_address or last_ip_address (server-set from
+// RegisterOptions.IP only), is_onboarded (application-owned onboarding
+// state) or must_change_password (set by an administrator or the
+// require-password-change command). The activation, reset, address proof,
+// handle and permission columns are never fillable at all.
+func (User) RegisterFillable() []string {
+	return []string{"name", "surname", "email", "password", "preferred_locale", "marketing_consent"}
+}
+
 // Hidden mirrors PHP User::$hidden for the secrets this model stores.
 func (User) Hidden() []string {
 	return []string{"password", "reset_password_code", "activation_code"}