line-items: one metadata row, agreed on by the write and the read

Three defects on one surface, all invisible while a transaction has exactly
one expense_metadata row — which is almost always.

1. ORDER BY (transaction_id = $1) DESC did the opposite of its comment.
   Postgres orders DESC as NULLS FIRST, and the row that merely POINTS at the
   transaction has a NULL transaction_id, so the clause written to prefer the
   owned row preferred the other one — in exactly the case it exists for.
   matchTransaction already spelled out NULLS LAST; now so do this and
   GET /api/transactions/[id]/order, which had no ORDER BY at all and could
   read a different row from the one an enrichment had just written.

2. replaceLineItems takes an optional metadataId. The match resolves a row
   through a platform-filtered join; re-picking independently here can land
   the write on a row the match ruled out.

3. previous_line_items on intake_batches is the undo baseline, recorded once
   per batch under COALESCE and read back. Returning whatever was just read
   hands a pantry retry its own write as the "previous" state, so undo
   restores the thing it undid. Migration 0034 is unapplied anywhere, so the
   column is added there rather than in a 0035.

prisma/migrations/0034_intake_batches/migration.sql

@@ -32,10 +32,20 @@ CREATE TABLE IF NOT EXISTS intake_batches (
   status         text NOT NULL,
   transaction_id integer,
   capture_id     integer,
+  -- The undo baseline: what `expense_metadata.line_items` held before this
+  -- batch first replaced it. Recorded here rather than returned from the write,
+  -- because pantry re-calls the enrich endpoint on every confirm and after a
+  -- timeout — a caller that hands back whatever it just read gives a retry its
+  -- OWN write as the "previous" state, and undo then restores the thing it was
+  -- meant to reverse. Written under COALESCE, so only the first snapshot sticks.
+  previous_line_items jsonb,
   created_at     timestamptz NOT NULL DEFAULT now(),
   resolved_at    timestamptz
 );
 
+-- For a database that already ran an earlier draft of this file.
+ALTER TABLE intake_batches ADD COLUMN IF NOT EXISTS previous_line_items jsonb;
+
 COMMENT ON TABLE intake_batches IS
   'One row per upload batch through POST /api/intake. Persisted before '
   'extraction so Slack taps can act on it later and re-shares are detected. '