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/migrations/0006_dvol_history_multi_asset.sql

@@ -0,0 +1,30 @@
+-- 0006_dvol_history_multi_asset.sql — promote dvol_history to multi-asset
+--
+-- Original schema (0001_init.sql) treated dvol_history as ETH-only:
+-- PRIMARY KEY (timestamp) and a column named eth_spot. With the
+-- orchestrator now snapshotting BTC in addition to ETH (commit
+-- e978a44), the table needs an asset dimension so we can store a
+-- DVOL/spot sample per asset per tick.
+--
+-- Forward-only. The 1028 existing rows are all ETH (the only writer
+-- was the ETH branch of market_snapshot_cycle) so we backfill
+-- asset='ETH' before swapping the table in place.
+
+CREATE TABLE dvol_history_v2 (
+    timestamp TEXT    NOT NULL,
+    asset     TEXT    NOT NULL,
+    dvol      NUMERIC NOT NULL,
+    spot      NUMERIC NOT NULL,
+    PRIMARY KEY (timestamp, asset)
+);
+
+INSERT INTO dvol_history_v2(timestamp, asset, dvol, spot)
+SELECT timestamp, 'ETH', dvol, eth_spot FROM dvol_history;
+
+DROP TABLE dvol_history;
+ALTER TABLE dvol_history_v2 RENAME TO dvol_history;
+
+CREATE INDEX idx_dvol_history_asset_ts
+    ON dvol_history(asset, timestamp DESC);
+
+PRAGMA user_version = 6;

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):

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),
             ),
         )