schema: previous_line_items on the model; say what the finish_reason rule is

Migration 0034 has the column and the Prisma model did not, so a regenerated
client and the database disagreed.

The extraction comment claimed only a finished reply is accepted while the code
accepts an absent finish_reason. The code is right — failing every call on a
provider that omits an optional field would be a certain outage in exchange for
a case that also requires syntactically complete JSON, and the harm a truncated
reply does is closed on the write side. The prose now states the rule as it is:
a reason that is PRESENT must be `stop`.

prisma/schema.prisma

@@ -283,6 +283,8 @@ model intake_batches {
   status         String
   transaction_id Int?
   capture_id     Int?
+  // The undo baseline, written once per batch (see 0034 and line-items.ts).
+  previous_line_items Json?
   created_at     DateTime  @default(now()) @db.Timestamptz(6)
   resolved_at    DateTime? @db.Timestamptz(6)
 }