Add options_editable option to select content_type fields

[greyskin]

Adds a new boolean field option, options_editable (default true), to select content type fields. When set to false, admin users editing a content entry can no longer add new options to that field via the Back Office's "Edit options" screen — the field itself remains fully usable, just not extensible.

This mirrors the existing ui_enabled option available on has_many/many_to_many fields, though it's a distinct flag rather than a reuse of ui_enabled, since ui_enabled already has an unrelated, broader meaning (whole-field visibility in the entry form) that doesn't fit this use case.

Motivation: some sites want a fixed, curated list of select options that only developers can change via Wagon/YAML, while others want admins free to expand the list from the Back Office. Currently this isn't configurable per-field.

Changes:

• lib/locomotive/custom_fields.rb — new options_editable field on the base Field model
• app/api/locomotive/api/forms/content_type_field_form.rb — whitelist the new attribute
• app/api/locomotive/api/entities/content_type_field_entity.rb — expose it for select fields
• app/api/locomotive/api/resources/content_type_resource.rb — accept it as an API param
• app/controllers/locomotive/custom_fields/select_options_controller.rb — enforce it (before_action, returns 403 if disabled) — this is the actual permission boundary, not just a UI nicety
• app/helpers/locomotive/custom_fields_helper.rb — hide the "Edit options" link when disabled
• Adds specs covering the new controller guard

Backward compatibility: defaults to true, so existing sites and fields are unaffected unless explicitly opted out.