The concurrency test only exercised the single-message append, which
allocates its position inside one statement. Replacing the batch
allocator's atomic `next_message_position` bump with a read-then-write
left every test passing — so the invariant that concurrent writers get
disjoint ranges was not actually covered on the path a turn takes.
Four concurrent three-message batches now assert twelve distinct
positions and that each batch's own messages stayed contiguous and in
order. Against the read-then-write version this fails on
`chat_messages_chat_position_key`, which is the constraint doing the
work rather than the application code.
Katia Bulatova
398a298561