feat(halacha-triage): quality-gated + prioritized review queue + metrics (#84)

Backend for the halacha approval-queue triage (#84). The keyboard UI, batch
actions and defer/reject (#84.4–6) already shipped; this adds the gating,
prioritization and metrics the queue was missing.

db.list_halachot — two opt-in triage controls:
  * exclude_low_quality (#84.1): drop items carrying ANY quality_flag
    (application / quote_unverified / truncated / non_decision / thin /
    nli_unsupported / near_duplicate) — they belong in a 'needs extraction fix'
    bucket, not the chair's approve queue.
  * order_by_priority (#84.3): active-learning order — negatively-treated
    first, then most-uncertain (lowest confidence), then oldest — instead of
    FIFO, so the highest-value decisions surface first.

halachot_pending (MCP) — now gated + prioritized BY DEFAULT; include_low_quality=
true reveals the needs-fix bucket. The agent review path benefits immediately.

GET /api/halachot — same two params, default OFF (non-breaking; the UI opts in).

metrics.halacha_backlog (#84.7) — splits pending into clean vs flagged, adds
deferred, reviewed_total, approve_ratio, and a pending_by_flag breakdown, so the
backlog distinguishes real review work from extraction noise.

Deferred (documented): #84.2 near-duplicate cluster cards and wiring the UI
fetch to the new params require frontend work + an api:types regen AFTER this
deploys (the new query params aren't in prod's OpenAPI until then) — a clean
follow-up. The backend fully supports both now.

Verified against the live DB (read-only):
- pending 177 → gated-clean 110, 0 flagged items leak into the clean queue.
- priority order surfaces the lowest-confidence items first (0.55, 0.55, ...).
- backlog: pending_clean=110 / pending_flagged=67 / approve_ratio=0.916,
  pending_by_flag={nli_unsupported:59, quote_unverified:3, thin:3, truncated:2}.
- pytest tests/test_halacha_quality.py — 52 passed (no regression).

Invariants: G1 (gate at source — SQL filter, not post-hoc); G2 (no parallel
path — same list_halachot); §6 (flagged items routed to a bucket, never dropped).

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

web/app.py

@@ -6031,7 +6031,13 @@ async def halachot_list(
     practice_area: str = "",
     limit: int = 200,
     offset: int = 0,
+    exclude_low_quality: bool = False,
+    order_by_priority: bool = False,
 ):
+    """List halachot. ``exclude_low_quality`` hides flagged items (#84.1) and
+    ``order_by_priority`` switches to the active-learning order (#84.3). Both
+    default off so existing callers are unaffected; the review-queue view opts
+    in."""
     cid: UUID | None = None
     if case_law_id:
         try:
@@ -6043,6 +6049,8 @@ async def halachot_list(
         review_status=review_status or None,
         practice_area=practice_area or None,
         limit=limit, offset=offset,
+        exclude_low_quality=exclude_low_quality,
+        order_by_priority=order_by_priority,
     )
     return {"items": rows, "count": len(rows)}