feat(backend): recurring expenses

[algirdasc]

Implements recurring payments/transactions in backend.

What’s included:

• New entity: RecurringExpense (schedule/template)
• New enum: RecurringFrequency (daily/weekly/monthly/yearly)
• Expense now optionally links to schedule + occurrence timestamp (idempotency via unique index)
• Generator service + console command: app:recurring-expenses:generate
• Create expense API now accepts optional recurringRule to create schedule on the fly
• Migration included

API shape (create expense):

• POST /api/expense supports optional "recurringRule": { frequency, interval, startAt, endAt }

Notes / follow-ups:

• This PR focuses on backend; UI changes can be added separately.
• Editing/canceling schedules and ‘edit single vs series’ not included yet.