fix(ocr): round-4 review — defensive poll + drop dead tracking columns

Round 4 review (PR #910), no blockers:
- poll(): a 2xx body with no `status` now fails fast (logged) instead of being
  treated as perpetually pending until the deadline; defensive page index
  (`p.get("index", i)`) so a malformed page degrades rather than KeyError-ing.
- Document on poll() that job_id is namespaced (embeds "/") so the gateway route
  must be a path-capture param (GET /v1/ocr/batch/{job_id:path}).
- Drop the vestigial `status` + `updated_at` columns from batch_ocr_jobs: a row
  only ever exists while pending (terminal jobs are deleted) and the live status
  comes from a fresh poll, so a stored mirror was permanently "pending" /
  redundant with submitted_at. Simplifies the migration, store, and dataclass.
- Tests: submit() ValueError on missing job_id; poll() missing-status → failed.

1653 unit tests pass; ruff + ty green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_batch_ocr_store.py

@@ -35,7 +35,6 @@ async def test_insert_then_get(store):
     job = await store.get(**_DOC)
     assert job is not None
     assert job.job_id == "mistral/j1"
-    assert job.status == "pending"
     assert job.submitted_at > 0