feat(halachot): Phase 5 — canonical panel UI + instances accordion (V41)

UI changes to halacha-review-panel.tsx:
- instance_type badge (עיקרון מקורי / ציטוט / יישום) in meta row
- "מוזכר ב-N פסיקות" pill when instance_count > 1
- CanonicalSection component: canonical_statement (view + edit), instances accordion

Backend:
- list_halachot SQL: adds canonical_id, instance_type, canonical_statement,
  instance_count via LEFT JOIN canonical_halachot
- list_canonical_instances(canonical_id) → compact rows for accordion
- GET /api/canonical-halachot/{canonical_id}/instances endpoint
- PATCH /api/halachot/{id}: canonical_statement propagates to canonical_halachot
- HalachaUpdateRequest: canonical_statement field
- useCanonicalInstances hook + CanonicalInstance type in precedent-library.ts

INV-G10 (chair gate): only the chair can edit canonical_statement (same
flow as rule_statement — PATCH /api/halachot/{id} with reviewer="דפנה").
G2: canonical data flows through canonical_halachot, not a parallel store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/app.py

@@ -6025,6 +6025,7 @@ class HalachaUpdateRequest(BaseModel):
     subject_tags: list[str] | None = None
     practice_areas: list[str] | None = None
     supporting_quote: str | None = None  # #133 — edited quote → re-verify + sync flag
+    canonical_statement: str | None = None  # V41 — updates canonical_halachot.canonical_statement
 
 
 class HalachaBatchReviewRequest(BaseModel):
@@ -7414,6 +7415,18 @@ async def halacha_equivalents_unlink(halacha_id: str, other_id: str):
     return {"ok": await db.unlink_equivalent_halachot(hid, oid)}
 
 
+# ── Canonical halachot — V41 ─────────────────────────────────────────────────
+
+@app.get("/api/canonical-halachot/{canonical_id}/instances")
+async def canonical_halacha_instances(canonical_id: str):
+    """All halachot instances sharing a canonical_id (V41 — used by the UI accordion)."""
+    try:
+        cid = UUID(canonical_id)
+    except ValueError:
+        raise HTTPException(400, "canonical_id לא תקין")
+    return {"instances": await db.list_canonical_instances(cid)}
+
+
 # ── Gold-set tagging (#81.7 / #81.8) ─────────────────────────────────────────
 
 class GoldsetSampleRequest(BaseModel):
@@ -7490,6 +7503,16 @@ async def halacha_update(halacha_id: str, req: HalachaUpdateRequest):
     )
     if not row:
         raise HTTPException(404, "הלכה לא נמצאה")
+    if req.canonical_statement is not None:
+        # V41: propagate to canonical_halachot; canonical_id not in RETURNING
+        # so we fetch it separately (one cheap indexed lookup).
+        _pool = await db.get_pool()
+        canon_id = await _pool.fetchval(
+            "SELECT canonical_id FROM halachot WHERE id=$1", hid
+        )
+        if canon_id:
+            from uuid import UUID as _UUID
+            await db.update_canonical_statement(_UUID(str(canon_id)), req.canonical_statement)
     return row