Adversarial review F6 (Low, confirmed): Channel.created_at: Instant was
drafted 'for the idle timeout' (spec §5.1) but the timer keys off
RtcSession.last_rtp_rx (and now last_rtp_rx after F4), never creation
time. The field was never read; the Instant import it pulled is dropped
with it.
Spec §5.1 already reconciled in 8680f2b (struct sketch + comment).
Spec §4.5 already specified 'no RTP' (60 s); the contradiction with
the old 5-min lineage is closed.
Refactor phase — full test suite + clippy green; no behavior change.
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