Plain sends `customer.externalId` as an explicit `null` rather than
omitting the key. The schema validated it with `z.string().optional()`,
which accepts `undefined` but rejects `null`, so every customer we don't
set an `externalId` for got a 400 instead of a card — while the rest
worked, which made it look intermittent.
`email`, `externalId` and `thread` are now `nullish`. One of
email/externalId is still required, and the route's existing email
fallback resolves these customers.
Three related fixes in the same path:
- The route returned `{ cards: [] }` when no user matched. Plain records
an integration error for any requested key it doesn't get back, so that
surfaced as a broken card rather than a hidden one. Every requested key
is now answered, with `components: null` where there's no data.
- The impersonation link is offered only when the customer matched on
`externalId` — a value we set ourselves. An email match is a weaker
claim, since the address on a Plain customer isn't verified and for
customers created outside our own writes it comes from whoever sent the
message. Email-matched customers get the account rows without a
one-click impersonation link.
- The not-found log recorded raw customer identifiers; it now keeps
presence flags only.
The schema and the response helper moved to
`app/utils/plainCustomerCards.ts` so they can be unit-tested without
pulling in the db and env modules.
## Testing
`app/utils/plainCustomerCards.test.ts` — 11 tests covering the null
shapes, the every-key-answered response, and the missing-vs-zero
distinction. Verified locally.
Split out of #4571, which bundled this with an unrelated impersonation
fix.
Iss