Welcome: pre-subscription funnel rename and member subscription-required screen

[paulocastellano]

Context

Split out of #204. Pre-pay ICP flow only — frees /onboarding for the later Onboarding module.

Goal

Rename the pre-Stripe ICP flow to Welcome, drop the social-connect gate before checkout, and hold unpaid members on a dedicated waiting screen.

In scope

• Routes/pages: /welcome/{persona|goals|referral-source}
• WelcomeController + FormRequests rename from Onboarding ICP requests
• Checkout CTA starts from the referral step (no subscribe/social gate before Stripe)
• /welcome/subscription-required for members of accounts without app access (poll until owner subscribes)
• Legacy /onboarding/* ICP URLs redirect to welcome equivalents (keeps /onboarding free for the checklist later)
• Welcome i18n + Welcome/PostHog events for this funnel
• EnsureAccountReady sends unpaid members to subscription-required (not through ICP steps they cannot finish)

Out of scope

• Post-subscription checklist UI / residual banner
• Changing /billing/processing destination to the checklist (Onboarding module)
• Trial days / promo codes / purchase-tracking hardening (Billing module)
• MCP settings

Independence

Safe to merge to main alone. Post-checkout continues to land where main lands today. Do not introduce the activation checklist under /onboarding in this issue.

Suggested test plan

• New signup walks persona → goals → referral → checkout CTA
• Legacy onboarding ICP URLs redirect to welcome
• Member without app access lands on subscription-required and can leave when owner subscribes
• Owner with app access is not forced through welcome
• Browser smoke: welcome walkthrough up to enabled checkout CTA