#797 send_signal was broken outright: jsonb = json operator error in dedup query

SignalService.send_signal dedup SELECT compares jsonb column signal_payload with a psycopg Json-typed parameter -> UndefinedFunction: operator does not exist: jsonb = json. EVERY send_signal call (any payload, including {}) threw. Found while smoke-testing the new signal wake (#792). Fixed: Json -> Jsonb for both dedup compare and INSERT. Confirms signals were never usable end-to-end.