Append a message to the bulletin board.

Returns 201 when the message is stored and any registered signal action ran. When a signal action hook fails (for example a blocker whose clearance gate did not materialize), the message is still on the append-only board and queued in the board's retry outbox, but the action is not complete: the response is 202 rather than 201 so the caller can tell "stored and acted on" from "stored, action pending retry" (#2648).