Update API references from OpenExchange to ForexRateAPI in code and documentation

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

lib/state/rent_controller.dart

@@ -13,7 +13,7 @@ import 'package:rental_income_tracker/services/storage_service.dart';
 import 'package:rental_income_tracker/services/time_service.dart';
 import 'package:timezone/timezone.dart' as tz;
 
-const String openExchangeApiKey = 'REPLACE_WITH_YOUR_OPENEXCHANGE_API_KEY';
+const String forexRateApiKey = 'REPLACE_WITH_YOUR_FOREXRATE_API_KEY';
 
 class RentTableRow {
   RentTableRow({
@@ -40,19 +40,19 @@ class RentTableRow {
 class RentController extends ChangeNotifier {
   RentController({
     StorageService? storageService,
-    OpenExchangeService? exchangeService,
+    ForexRateApiService? exchangeService,
   }) : _storageService = storageService ?? StorageService(),
        _exchangeService =
            exchangeService ??
-           OpenExchangeService(
+           ForexRateApiService(
              httpClient: http.Client(),
-             apiKey: openExchangeApiKey,
+             apiKey: forexRateApiKey,
            ) {
     _notificationService = NotificationService(_onNotificationAction);
   }
 
   final StorageService _storageService;
-  final OpenExchangeService _exchangeService;
+  final ForexRateApiService _exchangeService;
   late NotificationService _notificationService;
 
   AppSettings _settings = AppSettings.defaults();
@@ -146,12 +146,21 @@ class RentController extends ChangeNotifier {
         throw StateError('Set rent amount in Settings before marking paid.');
       }
 
+      final key = TimeService.monthKey(year, month);
+      if (_records.containsKey(key)) {
+        await _notificationService.cancelForMonth(year, month);
+        await _syncNotificationScheduleForCurrentMonth();
+        return;
+      }
+
       final paidAt = DateTime.now().toUtc();
+      final usd = _settings.rentUsd;
       final estPaidAt = tz.TZDateTime.from(paidAt, TimeService.est);
       final estDate = DateTime(estPaidAt.year, estPaidAt.month, estPaidAt.day);
-      final rate = await _exchangeService.fetchUsdToSekRate(estDate: estDate);
-      final usd = _settings.rentUsd;
-      final sek = usd * rate;
+      final conversion = await _exchangeService.convertUsdToSek(
+        estDate: estDate,
+        amount: usd,
+      );
       final onTime =
           assumeOnTime || TimeService.isOnTimeByDeadline(paidAt, year, month);
 
@@ -159,8 +168,8 @@ class RentController extends ChangeNotifier {
         year: year,
         month: month,
         usdAmount: usd,
-        sekAmount: sek,
-        usdToSekRate: rate,
+        sekAmount: conversion.result,
+        usdToSekRate: conversion.quote,
         paidAtUtc: paidAt,
         onTime: onTime,
         source: source,
@@ -194,9 +203,12 @@ class RentController extends ChangeNotifier {
           continue;
         }
 
-        final estDate = DateTime(year, month, 1);
-        final rate = await _exchangeService.fetchUsdToSekRate(estDate: estDate);
         final usd = _settings.rentUsd;
+        final estDate = DateTime(year, month, 1);
+        final conversion = await _exchangeService.convertUsdToSek(
+          estDate: estDate,
+          amount: usd,
+        );
 
         final paidAtUtc = tz.TZDateTime(
           TimeService.est,
@@ -210,8 +222,8 @@ class RentController extends ChangeNotifier {
           year: year,
           month: month,
           usdAmount: usd,
-          sekAmount: usd * rate,
-          usdToSekRate: rate,
+          sekAmount: conversion.result,
+          usdToSekRate: conversion.quote,
           paidAtUtc: paidAtUtc,
           onTime: true,
           source: 'backfill',