Push: retire or demote the polling worker once FCM is live #48

Part of #44 — Push notifications: real device push (FCM) to replace the WorkManager poll

Once FCM delivers, the poll should stop being the primary path.

• Keep it as a low-frequency reconciliation pass (it also repairs the unread count after missed
  messages), or remove it entirely — decide with measurements, not by assumption.
• Make sure the two paths cannot both notify for the same item (the persisted last-seen marker is
  the natural coordination point).
• Whatever is decided, the battery cost of the current interval should not survive unchanged.

Tests: no duplicate notifications with both paths enabled.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.