Delivery::Dispatch
Delivery::Dispatch — fans episodes out to subscribers per their delivery_mode.
eligible_targets is the pure heart of the module: given subscriber and
episode projections, it computes which (subscriber, episode) combinations
should actually receive a Telegram delivery. Backfill protection lives here:
only episodes published AFTER the user subscribed are delivered.
fanout (added in the next task) wires this to the real models + side
effects (spawn per target, call Notify or AudioSender by mode).