fix: exchange-specific volume rank APIs for OKX and full top20

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_hub_volume_rank_lib.py

@@ -1,6 +1,8 @@
 from datetime import datetime
 
 from hub_volume_rank_lib import (
+    CACHE_VERSION,
+    _okx_turnover_usdt,
     cache_needs_refresh,
     format_volume_quote,
     merge_exchange_rank,
@@ -26,6 +28,11 @@ def test_format_volume_quote():
     assert format_volume_quote(4500) == "4.50K"
 
 
+def test_okx_turnover_usdt():
+    qv = _okx_turnover_usdt({"volCcy24h": "100", "last": "50"})
+    assert qv == 5000.0
+
+
 def test_cache_needs_refresh_and_merge():
     cache = {"rank_date": "2026-06-05", "exchanges": {}}
     assert cache_needs_refresh(cache, expected_rank_date="2026-06-07") is True
@@ -41,3 +48,8 @@ def test_cache_needs_refresh_and_merge():
     )
     assert merged["exchanges"]["binance"]["items"][0]["symbol"] == "BTC/USDT"
     assert merged["rank_date"] == "2026-06-07"
+
+
+def test_stale_cache_version_forces_refresh():
+    cache = {"version": CACHE_VERSION - 1, "rank_date": "2026-06-07", "exchanges": {"okx": {"items": [{}]}}}
+    assert cache_needs_refresh(cache) is True