feat: review chat — ask the run why it concluded a finding

Read-only Q&A on the review screen, per finding and per run, answered from
the job's own artifacts (evidence, cluster, extraction, verification, Brain
merge, sheet index, cover reconciliation, job.log). It never mutates findings,
decisions, or the report.

Turns are logged job-locally (review/chat_log.jsonl, transcript at
/jobs/{id}/review-chat/log) and to a cross-job feedback store
(REVIEW_FEEDBACK_DIR), which now also receives review decisions with their
category/severity corrections.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0115gGtrSxXE9DKvS9XPFSoT

tests/conftest.py

@@ -0,0 +1,15 @@
+"""Shared test fixtures."""
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def isolated_feedback_store(tmp_path, monkeypatch):
+    """Keep the cross-job feedback store out of the real outputs directory.
+
+    Saving a review decision or asking a chat question appends to
+    config.REVIEW_FEEDBACK_DIR, which is process-wide rather than job-local.
+    Without this, running the suite would accumulate junk in backend/outputs.
+    """
+    monkeypatch.setattr("backend.config.REVIEW_FEEDBACK_DIR",
+                        str(tmp_path / "_feedback"))