fix: skip invoice.upcoming webhook to prevent NOT NULL violation

[yoshifumi-kondo]

Summary

• Remove invoice.upcoming from the event handler registry — this event contains a preview invoice without an id field, which causes a NOT NULL constraint violation on the invoices table's generated id column
• Add a defensive guard in processEvent() to skip any event whose data.object lacks an id, protecting against similar edge cases from future Stripe event types
• Add unit tests verifying the fix and a test fixture for invoice.upcoming

Fixes #102

Context

Stripe sends invoice.upcoming 7 days before subscription renewal. Unlike regular invoices, this is a preview object with no id. The sync engine's invoices table uses:

id TEXT GENERATED ALWAYS AS ((_raw_data->>'id')::TEXT) STORED NOT NULL

When _raw_data has no id key, this evaluates to NULL, violating the PRIMARY KEY constraint and returning HTTP 500 to Stripe.

Changes

File	Change
packages/sync-engine/src/stripeSync.ts	Remove invoice.upcoming handler + add id-less event guard
README.md / docs/index.md	Mark invoice.upcoming as handled (acknowledged & skipped)
packages/sync-engine/src/stripeSync-invoice-upcoming.test.ts	4 unit tests
packages/fastify-app/src/test/stripe/invoice_upcoming.json	Test fixture
packages/fastify-app/src/test/webhooks.test.ts	Add fixture to test suite

Test plan

• Unit tests pass (pnpm vitest run stripeSync-invoice-upcoming.test.ts — 4/4 passing)
• Fastify webhook integration test passes with invoice_upcoming.json fixture
• Verify no regression on existing invoice event handling (invoice.paid, invoice.created, etc.)