feat(billing): requireQuota middleware for plan-based limits

[PierreBrisorgueil]

Context

Downstream projects need to enforce per-plan limits (e.g. max scraps, executions/month).

Spec

• New middleware: requireQuota(resource, action)
• Reads quota config from config.billing.quotas[plan][resource][action]
• Checks current usage via BillingUsage service
• Returns 429 { type: 'QUOTA_EXCEEDED', resource, action, limit, current, upgradeUrl } if over limit
• Org without subscription → treated as free
• Subscription past_due → treated as free

Config format (each project provides its own)

quotas: {
  free: { scraps: { create: 3, execute: 100 } },
  starter: { scraps: { create: 20, execute: 2000 } },
  pro: { scraps: { create: Infinity, execute: Infinity } }
}

Acceptance

• Middleware + tests
• Works with any project's quota config
• Depends on: BillingUsage model