- Move sentinel from -1 to -100 (outside the negate range) so moves from
position 0 no longer collide with negated row at -1
- Pull count check + position validation inside sql.begin in POST so two
concurrent inserts can't both pass the max-5 guard
- Wrap movePane + state UPDATE in a single transaction in PATCH so partial
failures roll back consistently
Adds /api/sessions/:id/panes (GET, POST), /api/panes/:id (PATCH, DELETE)
with transactional position-shift logic (negate-and-restore pattern to
avoid UNIQUE collisions). Max 5 panes per session enforced.
Sessions.POST now creates the session and a default Chat pane at position
0 atomically via sql.begin.
