feat(ocr): add unique constraint on (job_id, document_id)

Prevents the same document from being added to an OCR job twice.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/main/resources/db/migration/V28__add_unique_constraint_ocr_job_documents.sql

@@ -0,0 +1,2 @@
+ALTER TABLE ocr_job_documents
+    ADD CONSTRAINT uq_ocr_job_document UNIQUE (job_id, document_id);