feat: changing event exchange rate recalculates all expense converted amounts

- New RecalculateExpenses DB function updates all expenses for an event
- Expenses in different currency: converted = ROUND(amount * new_rate, 2)
- Expenses already in base currency: left unchanged
- Called from UpdateEvent handler after saving the new event rate

internal/database/db.go

@@ -307,6 +307,23 @@ func UpdateEvent(db *sql.DB, id, baseCurrency string, exchangeRate float64) erro
 	return err
 }
 
+// RecalculateExpenses updates all expense converted_amounts for an event
+// using the new exchange rate. Expenses already in the base currency are left unchanged.
+func RecalculateExpenses(db *sql.DB, eventID, baseCurrency string, exchangeRate float64) error {
+	_, err := db.Exec(
+		`UPDATE expenses SET
+		 converted_amount = CASE WHEN currency != ? THEN ROUND(amount * ?, 2) ELSE amount END,
+		 base_currency = ?
+		 WHERE event_id = ?`,
+		baseCurrency, exchangeRate, baseCurrency, eventID,
+	)
+	if err != nil {
+		log.Printf("ERROR [%s] database: RecalculateExpenses(%s): %v",
+			time.Now().Format(time.RFC3339), eventID, err)
+	}
+	return err
+}
+
 // ---------------------------------------------------------------------------
 // Expense queries
 // ---------------------------------------------------------------------------

internal/handlers/events.go

@@ -362,6 +362,12 @@ func (h *EventHandler) UpdateEvent(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Recalculate all existing expense converted amounts with the new rate.
+	if err := database.RecalculateExpenses(h.DB, eventID, baseCurrency, exchangeRate); err != nil {
+		log.Printf("ERROR [%s] handlers: UpdateEvent: recalc expenses: %v", time.Now().Format(time.RFC3339), err)
+		// Non-fatal — the event was updated, but expenses may have stale conversions.
+	}
+
 	w.Header().Set("HX-Redirect", "/dashboard")
 	w.WriteHeader(http.StatusOK)
 }