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

backend/.env.example

@@ -51,6 +51,21 @@ AGENT_REVIEW_AUDIT_SAMPLE=5
 # Allow future cross-job review-feedback aggregation to include source_text/images/comments
 REVIEW_AGGREGATE_INCLUDE_TEXT=false
 
+# Review chat: read-only Q&A about findings and coverage on the review screen.
+# It explains what the run did from the job's artifacts; it never changes a
+# finding, a decision, or the report.
+ENABLE_REVIEW_CHAT=true
+# Model for chat answers (blank inherits TEXT_MODEL)
+REVIEW_CHAT_MODEL=
+REVIEW_CHAT_MAX_TOKENS=4096
+# Prior turns replayed into a thread's prompt
+REVIEW_CHAT_HISTORY_TURNS=6
+# Max job.log lines searched into the chat's context bundle
+REVIEW_CHAT_LOG_LINES=40
+REVIEW_CHAT_MAX_QUESTION_CHARS=2000
+# Cross-job store for review decisions + chat turns (blank = backend/outputs/_feedback)
+REVIEW_FEEDBACK_DIR=
+
 # Pipeline tuning
 PDF_DPI=100
 MAX_PAGES=60