feat(ocr): discard abandoned capture batches

A bad scan, the wrong PDFs or a duplicate upload used to leave a batch
sitting in READY_FOR_REVIEW forever, because the only exits were confirm
(posts to the books) or rejecting every page one at a time. Add a
DISCARDED terminal status to both OCR domains and a single endpoint per
domain that rejects every page still pending in one shot.

Discarding is refused once anything has landed: statements once a page is
POSTED, policies once a page is APPLIED. Those batches did real work and
have to be settled page by page.

- POST /statements/batches/:id/discard
- POST /policy-ocr/batches/:id/discard
- shared DiscardBatchCard on both review screens, gated the same way

packages/database/prisma/migrations/20260801120000_ocr_batch_discarded/migration.sql

@@ -0,0 +1,12 @@
+-- Add DISCARDED to both OCR batch status enums. Staff can now abandon a
+-- pending review queue outright instead of leaving it stuck in
+-- READY_FOR_REVIEW forever (rejecting every page never closed the batch).
+-- Purely additive: no existing row changes value.
+
+-- AlterTable
+ALTER TABLE `policy_ocr_batches`
+    MODIFY `status` ENUM('UPLOADED', 'PROCESSING', 'READY_FOR_REVIEW', 'COMPLETED', 'FAILED', 'DISCARDED') NOT NULL DEFAULT 'UPLOADED';
+
+-- AlterTable
+ALTER TABLE `statement_batches`
+    MODIFY `status` ENUM('UPLOADED', 'PROCESSING', 'READY_FOR_REVIEW', 'COMPLETED', 'FAILED', 'DISCARDED') NOT NULL DEFAULT 'UPLOADED';