The cron->session mirror now fires ONLY for the delivery target that
equals the job's origin (platform+chat_id[+thread_id]). A job created
from a live gateway chat stamps that chat as origin, and that session is
guaranteed to exist (it is the conversation the user scheduled the job
in). Fan-out / broadcast / home-channel-fallback targets are never
mirrored: they are not a continuation of a conversation and may have no
session at all.
This makes the prior 'cold-start session seeding' concern a non-case by
construction: when the mirror semantically applies the session exists;
when none exists the target was never the origin, so we no-op.
Adds _target_matches_origin() + origin-scoping tests (exact match,
other-chat/other-platform/no-origin rejection, thread scoping, fan-out
mirrors only the origin target).
c06ceb3232