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/market_snapshot_cycle.py

@@ -181,19 +181,18 @@ async def collect_market_snapshot(
             try:
                 with transaction(conn):
                     ctx.repository.record_market_snapshot(conn, record)
-                    # Mirror ETH spot+DVOL into dvol_history so monitor_cycle's
-                    # return_4h lookup has local samples even in data-only mode.
-                    if (
-                        record.asset == "ETH"
-                        and record.spot is not None
-                        and record.dvol is not None
-                    ):
+                    # Mirror spot+DVOL into dvol_history (per asset) so
+                    # monitor_cycle's return_4h lookup has local samples even
+                    # in data-only mode. dvol_history enforces NOT NULL on
+                    # dvol/spot so skip if either is missing.
+                    if record.spot is not None and record.dvol is not None:
                         ctx.repository.record_dvol_snapshot(
                             conn,
                             DvolSnapshot(
                                 timestamp=record.timestamp,
+                                asset=record.asset,
                                 dvol=record.dvol,
-                                eth_spot=record.spot,
+                                spot=record.spot,
                             ),
                         )
             finally: