ibkr: emit ibkr_cash_balance{currency, account} per CashReport row

Each daily run now pushes one Pushgateway metric per currency row from
the Flex Activity Query's CashReport section (typically BASE_SUMMARY
aggregate + one row per held currency). Makes dormant-account balance
checks trivial and adds a Grafana surface for cash drift alerting.

Requires the Activity Flex Query in IBKR Client Portal to have the
CashReport section enabled.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

broker_sync/cli.py

@@ -310,6 +310,16 @@ def ibkr(
                         float(drift),
                     )
                 )
+            # Cash balances (one row per currency from CashReport, plus a
+            # BASE_SUMMARY row consolidated in account base currency).
+            for currency, ending_cash in provider.cash_balances():
+                drift_metrics.append(
+                    (
+                        "ibkr_cash_balance",
+                        {"currency": currency, "account": "ibkr-uk"},
+                        float(ending_cash),
+                    )
+                )
             drift_metrics.append(
                 ("ibkr_sync_last_success_timestamp_seconds", {}, float(time.time()))
             )

broker_sync/providers/ibkr.py

@@ -257,3 +257,20 @@ class IBKRProvider:
             )
             out.append((symbol, Decimal(str(pos.position))))
         return out
+
+    def cash_balances(self) -> list[tuple[str, Decimal]]:
+        """Return ``[(currency, ending_cash), ...]`` from the CashReport.
+
+        Includes the ``BASE_SUMMARY`` aggregate row (account base currency
+        consolidated) plus any per-currency rows. Empty list if no
+        CashReport section in the Flex query or before first ``fetch()``.
+        """
+        if self._last_response is None:
+            return []
+        stmt = self._last_response.FlexStatements[0]
+        out: list[tuple[str, Decimal]] = []
+        for row in stmt.CashReport or []:
+            if row.endingCash is None or row.currency is None:
+                continue
+            out.append((str(row.currency), Decimal(str(row.endingCash))))
+        return out