3677e4aace
FW-01: job_subscriber.py now has on_disconnect callback (5-arg paho v2 signature), reconnect_delay_set(1,16) for exponential backoff, and with_retry-wrapped initial connect (5 attempts). paho loop_start() handles auto-reconnect internally. FW-05: publish_event.py signs payloads with HMAC-SHA256 using auth_token as key (replaces plaintext token in wire). mqtt_common.py adds verify_hmac() helper using hmac.compare_digest (timing-safe). job_subscriber.py validates incoming events via verify_hmac. PoC mode (auth_token=None) skips verification — backward compatible. Reviewed by agy-existing (PASS) and claude-existing (FAIL: on_disconnect 4-arg signature → fixed to 5-arg matching paho v2 CallbackAPIVersion).