Implement activity-scoped Live Activity update-token fanout

[mithileshchellappan]

Summary

Implement Pushboy storage and fanout support for activity-scoped Live Activity update tokens.

Admin-api and node-server will send activityId with future update-token registrations, but Pushboy still needs to persist token/activity associations and gate update/end fanout by activity before this can fix over-fanout.

Required contract changes

• /v1/live-activity/tokens must accept optional activityId.
• Token write and token/activity association write must be transactional.
• Add live_activity_token_activities(activity_id, token_id, created_at, last_seen_at, source).
• Keep start fanout as topic broadcast.
• Gate update and future end fanout behind LIVE_ACTIVITY_SCOPED_UPDATE_FANOUT, default off.
• When LIVE_ACTIVITY_SCOPED_UPDATE_FANOUT=false, keep current broadcast fanout behavior but emit shadow metrics comparing broadcast candidates vs associated candidates for the job activityId.
• When LIVE_ACTIVITY_SCOPED_UPDATE_FANOUT=true, update/end fanout should select only tokens associated with the job activityId.
• FCM Live Activity tokens should also be associated to activityId from successful start dispatch recipients, because Android receives the usable update token after start.
• APNs ExpiredToken must invalidate Live Activity tokens.
• APNs logs must not print raw token values.

Acceptance criteria

• Registering a token with { activityId } creates or refreshes the token/activity row in the same transaction as the token write.
• Re-registering the same token for a later activity updates last_seen_at and creates/refreshes the later activity association without breaking prior data needed for diagnostics.
• start jobs still use topic fanout.
• With scoped fanout off, operators can see broadcast-candidate count and activity-associated-candidate count per update/end job.
• With scoped fanout on, update/end jobs dispatch only to tokens associated with that exact activityId.
• APNs ExpiredToken outcomes remove/disable stale LA tokens so they do not keep participating in fanout.
• No raw APNs/FCM token is logged by provider send or outcome paths.