Add round-robin posting mode

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.

internal/storage/schema.go

@@ -40,6 +40,12 @@ CREATE TABLE IF NOT EXISTS post_log (
     posted_at     INTEGER NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS round_robin_state (
+    id           INTEGER PRIMARY KEY CHECK (id = 1),
+    last_source  TEXT,
+    last_tick_at INTEGER NOT NULL DEFAULT 0
+);
+
 CREATE TABLE IF NOT EXISTS reactions (
     id         INTEGER PRIMARY KEY AUTOINCREMENT,
     post_guid  TEXT NOT NULL,