refactor: stream NEMP announcements via dedicated drain queue

Replace the timer-event + polling-task + pipe architecture with a
streaming model: polling_task uses as_completed to queue announcements
as mods finish, and a separate drain task sends them at 1 msg/2s.

- Remove nemp_timer_event and TIME_EVENT_NAME entirely
- Add announcement_drain_task draining an asyncio.Queue at safe rate
- Extract _announce_mod_update helper (queue instead of direct send)
- polling_task now handles all end-of-cycle bookkeeping inline
- Crash handling: notify staff channel, then re-raise
- start_polling cleans up leftover tasks from previous crashes
- Fix pre-existing bug: log message printed release_version instead
  of last_release in "matches current release" branch

Author: Pyker