Add test coverage for billing sync + stripe-purchases worker

[InfinityBowman]

Context

Two gaps in billing test coverage worth filling.

1. Unit tests for syncStripeSubscription

New command at packages/workers/src/commands/billing/syncStripeSubscription.ts
has real branching that should be locked down. Add
packages/workers/src/commands/billing/__tests__/syncStripeSubscription.test.ts
covering:

• Stripe returns no subs, no existing DB row → returns {status: 'none'}, no write
• Stripe returns no subs, existing active row → row marked canceled with endedAt
• Stripe returns no subs, existing already-canceled row → no-op (idempotent)
• Stripe returns a sub, existing row → row updated with canonical Stripe state
• Stripe returns a sub, no existing row, metadata has orgId → new row inserted
• Stripe returns a sub, no existing row, no orgId → returns without writing

Pattern is already established in packages/workers/src/routes/billing/__tests__/.
Stripe is globally mocked in test setup; D1 seeded via resetTestDatabase.
Estimate: ~30 min.

2. Stand up test setup for packages/stripe-purchases

The stripe-purchases worker (handles checkout.session.completed →
orgAccessGrants) has zero test coverage. The duplicate billing webhook
tree in packages/workers previously carried 5 test files that exercised the
main-worker copy — but that copy was dead in prod, and it has since been
removed.

Tasks:

• Add vitest + @cloudflare/vitest-pool-workers config to packages/stripe-purchases
• Port the deleted tests and adapt them to the live code paths there:
  • processCheckoutSession (grant creation, grant extension, idempotency via
    stripeCheckoutSessionId unique constraint)
  • Customer handlers (customer.updated, customer.deleted)
  • Invoice handlers (dunning flow)
  • Payment intent handlers
  • Two-phase ledger trust model (signature verification, dedupe by
    payload hash + event ID)

Estimate: 2–3 hours.

Out of scope

• Route-level test for POST /sync-after-success — thin plumbing
• Frontend useEffect test — low value, covered by existing
  billing-flow.spec.ts e2e

[coderabbitai]

🔗 Related PRs

#214 - 212 stripe better auth [merged]
#239 - 238 ai agent readiness [closed]
#253 - 252 stripe hardening [merged]
#261 - 260 better checkout flow [merged]
#394 - swap to queue for emails [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!