Virtual Allocate

Process virtual pot allocations on payday.

For each rule linked to a budget category:

1. Find the matching budget for the current user. PIS-off savings / investments rules arrive without a budget row: that tracking row is created. With ?mode=propose a SPENDING category with no budget is not created here — the rule comes back as "needs_budget" and the user confirms the budget from the proposal the rules PUT returned (EQ-72 ruling 4). An older client gets the budget created as before.
2. Reset period_start_date to today (new period)
3. Mark virtual_pot_funded_at = now
4. Return confirmation

monthly_limit is never changed here. No money moves — this is purely tracking.