Previously 'hermes debug share' uploads only got DELETEd when the user
ran 'hermes debug share' again — opportunistic-sweep-on-invoke was the
only cleanup path. A user who uploaded once and never ran debug again
left pastes up until paste.rs's retention kicked in (which, empirically,
never actually expires them).
Hook _sweep_expired_pastes into the gateway cron ticker at the same
hourly cadence as the image/document cache cleanups. The opportunistic
sweep in 'hermes debug share' stays as a fallback for CLI-only users
who never start the gateway.
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