Launch custom code

[lylo]

Removes the feature toggle from #1146, so head and body code are available to every subscriber rather than to flagged users only.

• config/features.rb: the custom_code flag is gone
• Settings: the Scripts section is no longer conditional, and custom_code_editable? is inlined into custom_code_params now that it only checks the subscription
• Marketing: the custom code rows on the home page, the five comparison pages, and llms.txt are unconditional
• Tests: drops the flag from the joel fixture and removes the "without the feature flag" case, which no longer has anything to assert

Behaviour change: every subscriber gets head and body code, and the feature is advertised publicly. Nothing else moves – free and trial users are still limited to Custom CSS and the footer.

Deploy note: llms.txt and the comparison pages are page-cached and their output changes here, so this deploy needs rails cache:expire_static_pages, plus a Cloudflare purge for /.

Based on #1146 – merge that first.