refactor: rewrite scheduler as functional

packages/backend/src/apps/scheduler/common/cron-times.ts

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+const cronTimes = {
+  everyHour: '0 * * * *',
+  everyHourExcludingWeekends: '0 * * * 1-5',
+  everyDayAt: (hour: number) => `0 ${hour} * * *`,
+  everyDayExcludingWeekendsAt: (hour: number) => `0 ${hour} * * 1-5`,
+  everyWeekOnAndAt: (weekday: number, hour: number) => `0 ${hour} * * ${weekday}`,
+  everyMonthOnAndAt: (day: number, hour: number) => `0 ${hour} ${day} * *`,
+};
+
+export default cronTimes;

packages/backend/src/apps/scheduler/common/get-date-time-object.ts

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+import { DateTime } from 'luxon';
+
+export default function getDateTimeObjectRepresentation(dateTime: DateTime) {
+  const defaults = dateTime.toObject();
+
+  return {
+    ...defaults,
+    ISO_date_time: dateTime.toISO(),
+    pretty_date: dateTime.toLocaleString(DateTime.DATE_MED),
+    pretty_time: dateTime.toLocaleString(DateTime.TIME_WITH_SECONDS),
+    pretty_day_of_week: dateTime.toFormat('cccc'),
+    day_of_week: dateTime.weekday,
+  };
+}

packages/backend/src/apps/scheduler/common/get-next-cron-date-time.ts

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+import { DateTime } from 'luxon';
+import cronParser from 'cron-parser';
+
+export default function getNextCronDateTime(cronString: string) {
+  const cronDate = cronParser.parseExpression(cronString);
+  const matchingNextCronDateTime = cronDate.next();
+  const matchingNextDateTime = DateTime.fromJSDate(matchingNextCronDateTime.toDate());
+
+  return matchingNextDateTime;
+};