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/runtime/monitor_cycle.py

@@ -173,8 +173,8 @@ async def _fetch_return_4h(ctx: RuntimeContext, *, now: datetime) -> Decimal:
     conn = connect_state(ctx.db_path)
     try:
         row = conn.execute(
-            "SELECT timestamp, eth_spot FROM dvol_history "
-            "WHERE timestamp <= ? AND timestamp >= ? "
+            "SELECT timestamp, spot FROM dvol_history "
+            "WHERE asset = 'ETH' AND timestamp <= ? AND timestamp >= ? "
             "ORDER BY timestamp DESC LIMIT 1",
             (cutoff.isoformat(), floor.isoformat()),
         ).fetchone()
@@ -239,7 +239,7 @@ async def run_monitor_cycle(
         with transaction(conn):
             ctx.repository.record_dvol_snapshot(
                 conn,
-                DvolSnapshot(timestamp=when, dvol=dvol, eth_spot=spot),
+                DvolSnapshot(timestamp=when, asset="ETH", dvol=dvol, spot=spot),
             )
         positions = ctx.repository.list_positions(conn, status="open")
     finally: