fix: revert heartbeat_interval change breaking docket worker (#669)

The 30s heartbeat_interval added in #653 broke background task execution.
Likes scheduled since Dec 29 deploy have null atproto_like_uri.

Reverts the Docket heartbeat_interval back to default (2s).

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Co-authored-by: Claude Opus 4.5 <[e-mail]>

backend/src/backend/_internal/background.py

···
     async with Docket(
         name=settings.docket.name,
         url=settings.docket.url,
-        # default 2s is for systems needing fast worker failure detection
-        # with our 5-minute perpetual task, 30s is plenty responsive
-        heartbeat_interval=timedelta(seconds=30),
     ) as docket:
         _docket = docket