Quality-review polish on Task 8:
1. InviteUnavailableError gains a typed public readonly `reason` field
(the union 'not found' | 'revoked' | 'expired' | 'exhausted'). Task 11
route handler can switch on the discriminant to produce user-facing
copy without parsing the message string.
2. The original "aborts transaction if insertUser throws" test was
vacuous — the callback threw before any DB write, so SQLite ROLLBACK
never fired and the post-conditions were trivially true. Replaced
with two tests: one that explicitly validates the synchronous
short-circuit (no DB work happens at all), and a second that writes
a real users row inside the callback then throws AFTER the write,
proving the SQLite ROLLBACK actually reverts the in-callback write.
Jannis Braun
b775e3bcc9