feat(data): mirror ETH spot+DVOL in dvol_history dal market_snapshot

Popola dvol_history dentro la stessa transazione di market_snapshots,
così lo storico è disponibile anche in modalità data-only (STRATEGY=false).
Evita il warm-up vuoto di return_4h quando si abilita la strategia: il
monitor_cycle trova subito i campioni locali invece di dipendere dal
fallback Deribit get_historical.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_market_snapshot_cycle.py

@@ -164,3 +164,48 @@ async def test_returns_zero_for_empty_assets(tmp_path: Path) -> None:
     ctx = _ctx(tmp_path)
     n = await collect_market_snapshot(ctx, assets=(), now=_now())
     assert n == 0
+
+
+def _read_dvol_history(ctx: MagicMock) -> list[dict]:
+    import sqlite3
+
+    conn = connect(ctx.db_path)
+    conn.row_factory = sqlite3.Row
+    try:
+        rows = conn.execute(
+            "SELECT * FROM dvol_history ORDER BY timestamp"
+        ).fetchall()
+    finally:
+        conn.close()
+    return [dict(r) for r in rows]
+
+
+@pytest.mark.asyncio
+async def test_eth_snapshot_mirrors_into_dvol_history(tmp_path: Path) -> None:
+    ctx = _ctx(tmp_path)
+    await collect_market_snapshot(ctx, assets=("ETH", "BTC"), now=_now())
+    rows = _read_dvol_history(ctx)
+    assert len(rows) == 1
+    assert Decimal(str(rows[0]["dvol"])) == Decimal("55")
+    assert Decimal(str(rows[0]["eth_spot"])) == Decimal("3000")
+
+
+@pytest.mark.asyncio
+async def test_btc_only_snapshot_does_not_touch_dvol_history(
+    tmp_path: Path,
+) -> None:
+    ctx = _ctx(tmp_path)
+    await collect_market_snapshot(ctx, assets=("BTC",), now=_now())
+    assert _read_dvol_history(ctx) == []
+
+
+@pytest.mark.asyncio
+async def test_eth_snapshot_skips_dvol_history_when_dvol_missing(
+    tmp_path: Path,
+) -> None:
+    ctx = _ctx(tmp_path)
+    ctx.deribit.latest_dvol = AsyncMock(side_effect=RuntimeError("no dvol"))
+    await collect_market_snapshot(ctx, assets=("ETH",), now=_now())
+    # market_snapshots row still persisted, but dvol_history must stay empty
+    # because its schema enforces NOT NULL on dvol/eth_spot.
+    assert _read_dvol_history(ctx) == []