Onboarding: post-subscription activation checklist, residual banner, and processing redirect

[paulocastellano]

Context

Split out of #204. Final module that wires the full post-pay activation path.

Prerequisites on main (merge order)

Prefer these already merged (each was independently shippable):

1. Sidebar shell
2. MCP settings (checklist step 1 UX)
3. Welcome (ICP moved off /onboarding)
4. Billing (trial + verified purchase tracking)

Goal

After Stripe, owners land on a single-page activation checklist (MCP → social → first post), with a residual sidebar banner until done, realtime progress, and safe completion/skip semantics.

In scope

• Account schema: onboarding_completed_at, onboarding_dismissed_at, onboarding_skipped_steps + data backfill for accounts that already have hasAppAccess
• ResolveOnboardingStatus + OnboardingController activation endpoints (sync / per-step skip / complete)
• Single-page checklist UI; MCP skippable; social + first post required
• Observers + OnboardingStatusUpdated Echo broadcasts (+ poll fallback)
• Residual banner in the app sidebar (SidebarOnboarding) linking back to /onboarding
• /billing/processing sends owners to /onboarding (this is where that redirect finally flips)
• Onboarding PostHog events; celebration / terminal-state guards
• Feature + browser tests for skip clearing residual, member paths, etc.

Out of scope

• Re-doing Welcome ICP (already on /welcome)
• Re-doing trial/promo config (already in Billing)
• Re-building MCP settings page (link/reuse existing)

Independence note

This module is the only one that intentionally depends on the others for a complete funnel. It should still degrade gracefully if MCP settings UI is missing (connect via existing OAuth paths), but do not merge before Welcome has vacated /onboarding.

Suggested test plan

• Owner after checkout → processing → /onboarding
• Checklist steps complete via connect / first post; MCP skip works; residual clears
• Existing app-access accounts backfilled as dismissed (no banner)
• Unpaid accounts keep a clean slate until they subscribe
• Echo + poll update residual/checklist; terminal states stop polling
• Browser: skip MCP clears residual; mobile sidebar path covered