Docs: Pro and Enterprise, and the go-live gate

[ShadowWalker2014]

• Rewrites Cloud billing for the two plans: Pro ($0 until your apps make $10,000 a month, then 0.5% of revenue above $10,000, never more than $999 a month) and Enterprise (custom pricing from $50,000 a year). It covers tracked revenue, Start Pro through Stripe Checkout, the go-live gate (building, grace, paused, active), what pauses and what never does, held webhooks and payment failures.
• Adds the plan_required (402) error to the API errors page, upgrade_url to the v2 error, a 402 response on every gated operation, and the new billing fields (gate, flags, pro_bill_usd, free_up_to_usd, plan none/pro/enterprise, checkout { "plan": "pro" }, delivery status held). The generators in scripts/openapi/ were edited and the YAML regenerated.
• Removes Cloud Free, Cloud Standard, "paid plans have not shipped" and self-hosting as a price option from the guides, FAQ, getting started, README, llms status lines, every blog post's About paragraph, blog CTAs ("Start for free") and three blog SVGs.
• Merge after the server PR with the go-live gate (rd-monetization, branch monetization-pro): CI's drift check reads revenuedot main, where upgrade_url does not exist yet.