Centralize day/timestamp helper utilities

[rickstaa]

We compute day IDs and start-of-day timestamps in multiple mappings (e.g., BroadcasterDay, TranscoderDay, createOrLoadDay, and TicketBroker handlers). It would be good to centralize this logic in helpers.ts (e.g., dayStart(timestamp) and dayId(timestamp)) and reuse those helpers across mappings.

Goal: ensure day-boundary math is consistent across all handlers, reduce duplicated logic, and make future changes easier and safer.