Disconnect not checked during backlog replay

The main live loop in event_stream() checks whether the client has disconnected on every iteration, so a client that goes away while idle is noticed promptly. The backlog replay loop -- which can iterate over up to the configured backlog length (1000 by default) of entries for a user reconnecting after a long time away -- has no such check at all. If a client disconnects partway through a large replay, event_stream() keeps formatting and yielding every remaining backlog entry to a connection that's already gone, for however long that takes, before it ever gets a chance to notice -- wasted server work under exactly the load pattern (a long-absent user with a full backlog) that matters most.

The backlog replay for loop in event_stream() has no disconnect check at all, unlike the live while True loop a few lines below it.

See practice_tickets/tests/test_ticket_11_disconnect_during_replay.py: seed a 6-entry backlog, read 1 entry, then flip the fake request's disconnected state to True -- the generator keeps yielding all 5 remaining entries instead of stopping within 1-2 more reads.

This matters most exactly when it's most expensive: a user reconnecting after being offline for a while -- the scenario this whole project is built to handle well -- is the same user whose backlog replay has the most entries to get through. A disconnect check that only exists in the live loop protects the cheap, common case and misses the expensive, less common one.

Look at the disconnect check at the top of the while True loop in event_stream() -- the backlog for loop just above it is missing the equivalent check. The check needs to go before doing the work to format and yield the next entry, not after, so it actually saves the wasted work.