feat(billing): bill workspaces under a parent workspace

[vklimontovich]

What

A workspace can now bill under a parent workspace. The child inherits the parent's
plan, and the parent's billing page consolidates usage for the whole group.

• Child workspace — /settings/billing shows a card linking to the parent's billing
  page instead of its own billing UI. The child inherits the parent's plan (limits,
  feature gating, pastDue), and no Stripe customer is created for it.
• Parent workspace — /settings/billing sums events and active syncs across the
  parent and all its children (summed totals only, no per-child breakdown).
• Standalone workspace — unchanged.

How

• The parent→child relation lives in ee-api (newjitsuee schema) in a new
  WorkspaceBillingParent table, with all hierarchy/consolidation logic in
  lib/billing-hierarchy.ts.
• getOrCreateCurrentSubscription returns the parent's subscription for a child,
  tagged with a billingParent marker.
• Consolidated usage rides on the existing /api/billing/settings endpoint via an
  opt-in ?withUsage=true flag — no new endpoints. The app-wide BillingProvider
  fetch stays light; only the billing page requests usage.
• Links are managed from a new Billing Parents page in the ee-api admin UI
  (/billing-parents), backed by /api/admin/billing-parents.
• Constraints: only one level of nesting (a parent cannot itself have a parent), and
  free workspaces cannot be joined.

Notes / follow-ups

• Requires prisma db push on the newjitsuee schema to create
  workspace_billing_parent.
• Quota enforcement / throttling on consolidated group usage is intentionally out of
  scope — grouped workspaces are paid, so the only enforcement that matters
  (pastDue) is inherited by children automatically.
• The "View detailed stat" drill-down (/settings/billing/details) still shows a
  single workspace, not the consolidated group.
• ee-api has no test harness, so billing-hierarchy validation is not unit-tested.

Verification

1. pnpm codegen, then pnpm --filter ee-api db:update-schema to create the table.
2. pnpm ui:dev; in the ee-api admin Billing Parents page, link a paid child under
   a paid parent.
3. Child /settings/billing shows the link card; parent /settings/billing shows
   summed usage. Self-link / two-level nesting / free-plan child are rejected.