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/repository.py

@@ -339,12 +339,13 @@ class Repository:
         self, conn: sqlite3.Connection, snapshot: DvolSnapshot
     ) -> None:
         conn.execute(
-            "INSERT OR REPLACE INTO dvol_history(timestamp, dvol, eth_spot) "
-            "VALUES (?,?,?)",
+            "INSERT OR REPLACE INTO dvol_history(timestamp, asset, dvol, spot) "
+            "VALUES (?,?,?,?)",
             (
                 _enc_dt(snapshot.timestamp),
+                snapshot.asset,
                 _enc_dec(snapshot.dvol),
-                _enc_dec(snapshot.eth_spot),
+                _enc_dec(snapshot.spot),
             ),
         )