feat(session): gate background session work PR 6

[MohamadJaara]

Goal

Apply the same session preparation boundary to background entry points before they access user data.

Changes

• gate notifications, receivers, services, workers, FCM, and sync entry points
• move push token registration into prepared-session worker execution
• skip or retry background work when preparation cannot complete safely
• cover notification, logout, and call service behavior with focused tests

Stack

• Epic branch: mo/epic/async-db-migration
• PR 1
• PR 2
• PR 3
• PR 4
• PR 5
• PR 6 of 7

Merge the stack bottom-up into the epic branch.