Bug: an unrecognised `customerStatus` locks a paying subscriber out of subscriber features

[Adron]

Found while implementing #58 (list invites), which fixed it on the lists side. :feature:documents
still has it.

The bug

CustomerStatus in :core:model maps an unrecognised customerStatus string to UNKNOWN, whose
isSubscriber is false.

:feature:documents's currentUserIsSubscriber() treats that as "not a subscriber" — so if the
server ever returns a status the app does not know (a future subscriber:lifetime, a renamed tier,
anything), a paying owner is locked out of their own invite form without the server ever being
asked. The gate is client-side, so there is no 403 to recover from; the feature simply is not
offered.

The fix

:feature:lists (PR #119) treats UNKNOWN as unreadable rather than as free, and fails
open — the server stays the real gate, which it always was. It has a regression test,
an unrecognised customer status is treated as unknown, not free.

Apply the same one-line change to :feature:documents's currentUserIsSubscriber() and add the
equivalent test.

Worth checking at the same time

Any other client-side subscriber gate in the codebase. :core:materialize (#11) and :feature:ai
(#4) both deliberately fail open already, so they are fine — but a grep for isSubscriber before
closing this would be worthwhile.

Why it was not fixed in #58's PR

Scope — :feature:documents had parallel work in flight at the time.