Add account balance anchors

internal/domain/model.go

@@ -31,6 +31,16 @@ type Account struct {
 	// broker exports no counterparty column, so deposits and withdrawals carry
 	// this IBAN instead and pair with the funding account like any transfer.
 	ReferenceIBAN string `json:"reference_iban,omitempty"`
+	// AnchorBalance is the bank's booked (CLBD) balance on AnchorDate, captured
+	// once from open banking after a sync. It fixes the start balance of a
+	// date-windowed history: the money that existed before the recorded rows is
+	// AnchorBalance less every movement booked through AnchorDate, so the
+	// account's real balance is computable without complete history. The bank's
+	// figure is stored verbatim — the start balance is derived, never stored —
+	// so importing older history later corrects the derivation by itself.
+	// Cash accounts only: a broker export carries its complete history.
+	AnchorBalance Money  `json:"anchor_balance,omitempty"`
+	AnchorDate    string `json:"anchor_date,omitempty"`
 	Active        bool   `json:"active"`
 }