[SSF-146] handle recurring donations

[amywng]

ℹ️ Issue

Closes SSF-146

📝 Description

Implemented logic for processing recurring donations - every day, the cron job calls the service to check whether any scheduled donation dates have passed, send an email to the FM (placeholder log for now), and reschedule the next occurrence.
handleRecurringDonations():

• fetches all donations and filters for ones with past dates in nextDonationDates
• for each past date, log a placeholder for sending an automated email to the FM
• decrements occurrencesRemaining and removes the date from nextDonationDates
• calculates the next scheduled date using recurrence and recurrenceFreq (in a helper called calculateNextDate()) and adds it to nextDonationDates if there are remaining occurrences
• handles cascading recalculation - if the replacement date is also in the past, it continues advancing and decrementing occurrencesRemaining until a future date is reached or occurrences run out

Added service tests to cover all cases

• I added an insertDonation() helper to easily insert a new donation with custom recurrence info

✔️ Verification

Tests pass