Exclude !post from the daily cap

Manual !post overrides were counted toward daily_cap_total, so a few
forced posts could starve the round-robin rotation for the rest of the
day. Tag forced rows in post_log and skip them in CountAllPostsInWindow
so the cap only meters the auto-rotation.

internal/storage/schema.go

@@ -25,7 +25,8 @@ CREATE TABLE IF NOT EXISTS post_log (
     channel       TEXT NOT NULL,
     event_id      TEXT,
     url_canonical TEXT,
-    posted_at     INTEGER NOT NULL
+    posted_at     INTEGER NOT NULL,
+    forced        INTEGER NOT NULL DEFAULT 0
 );
 
 CREATE TABLE IF NOT EXISTS round_robin_state (