Round-robin: rotate by channel instead of by source

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.

internal/storage/schema.go

@@ -42,7 +42,7 @@ CREATE TABLE IF NOT EXISTS post_log (
 
 CREATE TABLE IF NOT EXISTS round_robin_state (
     id           INTEGER PRIMARY KEY CHECK (id = 1),
-    last_source  TEXT,
+    last_channel TEXT,
     last_tick_at INTEGER NOT NULL DEFAULT 0
 );