Source-keyed rotation skewed toward whichever channel had the most
feeds (4 of 7 sources routed to politics, so politics dominated the
rotation). Channel-keyed rotation guarantees variety regardless of
feed counts.
Schema: round_robin_state.last_source -> last_channel, added via
addColumnIfMissing so existing DBs migrate in place.
One story per interval_hours (default 4), cycling through enabled sources
in config order. Empty sources are skipped and the pointer advances to
whichever source actually posted. State persists across restarts.
Duplicate-flagged stories now get a _duplicate sentinel channel so they
stay out of the rotation pool alongside _discarded.
