refactor(webapp): drop the dashboard agent's JSONB transcript column

The feature is unreleased and this branch is local, so there is no rolling
deploy in which an old instance still selects `chats.messages` — the usual
reason to keep a replaced column for one more release does not apply, and
after release the drop would cost two deploys. So it goes now.

With no data to preserve there is no backfill, and with no backfill there
is no such thing as a stored message without an id: `message_id` is
`NOT NULL` and the writers read `message.id` with no fallback, so a
malformed message fails the insert instead of being given an invented
identity by untested code.

Removing the column from the drizzle schema is also the enforcement that
nothing writes it — a surviving TypeScript reference stops compiling. The
scan for raw SQL naming the column found nothing to guard, so there is no
test for it.

The four idempotency invariants each get their own test against a real
table: a repeated message id creates no row and keeps its position; eight
genuinely concurrent appends take eight distinct positions, with a raw
insert past the query layer proving it is `UNIQUE (chat_id, position)`
doing the work; a controlled update changes one body and leaves identity,
position and every other row alone; and a mid-turn append survives the
turn's write, landing where it happened rather than at the end.

`dashboardAgentTranscriptMerge.test.ts` becomes
`dashboardAgentTranscriptStore.test.ts`: it no longer tests a merge, and
the ordering it asserted has deliberately changed.

Author: kathiekiwi

apps/webapp/test/dashboardAgentInvestigationSweepCard.test.ts

@@ -25,7 +25,7 @@ const { sweepDashboardAgentInvestigations } =
  * The transcript half of the sweep, without a container: settling the row is invisible
  * to the panel, which resolves a card from the chat's own `render_view` parts.
  *
- * The database half — that the closing card actually lands in `chats.messages` — is
+ * The database half — that the closing card actually lands in `chat_messages` — is
  * covered by `dashboardAgentInvestigationSweep.test.ts`, which needs Postgres.
  */