feat(state): dvol_history multi-asset (ETH+BTC) + backfill ETH legacy rows

Migration 0006 promuove dvol_history da PK=(timestamp) mono-ETH a
PK=(timestamp, asset), rinomina eth_spot -> spot, e backfilla con
asset='ETH' le righe storiche. market_snapshot_cycle ora scrive sia
per ETH che per BTC; monitor_cycle resta ETH-only via WHERE asset='ETH'
nella lookup di return_4h.

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

src/cerbero_bite/state/models.py

@@ -116,8 +116,9 @@ class DvolSnapshot(BaseModel):
     model_config = ConfigDict(extra="forbid")
 
     timestamp: datetime
+    asset: str  # "ETH", "BTC"
     dvol: Decimal
-    eth_spot: Decimal
+    spot: Decimal
 
 
 class MarketSnapshotRecord(BaseModel):