docs(billing): add ERRORS.md entry for price.metadata.planId empty in Stripe webhooks

Author: PierreBrisorgueil

ERRORS.md

@@ -27,3 +27,4 @@ Use this file as a compact memory of recurring AI mistakes.
 - [2026-03-15] cross-stack: changing a Node API without checking Vue E2E tests -> when modifying an endpoint Vue consumes, run Vue E2E tests before pushing
 - [2026-03-15] pr scope: batching multiple unrelated fixes in one PR -> one fix = one PR to isolate blast radius and reduce iteration loops
 - [2026-05-05] repository: Repository.update(doc) doing `new Model(doc).save()` rewrites the full document from in-memory state, silently clobbering any concurrent partial update that landed after the read -> always use `Model.updateOne({ _id }, { $set: ... })` or `findOneAndUpdate({ _id }, { $set: ... })` for partial updates to avoid race conditions; see comes-io/trawl_node#1115 comes-io/trawl_node#1116 comes-io/trawl_node#1118 + pierreb-devkit/Node#3605
+- [2026-05-31] billing/stripe: reading `price.metadata.planId` in `customer.subscription.updated` webhook handler -> field is EMPTY in real Stripe webhook payloads (planId lives on the Product, not the Price); use a `priceId → plan` map built at boot from `config.stripe.prices` instead; see pierreb-devkit/Node#3742