Three bugs were letting mechanism/terrain/radar contacts ping-pong
between :complete and :queued on every backfill cron:
1. enqueue_for_contact/2 now filters out types that are already
:complete on the contact before building jobs. Previously
mark_status!/3 demoted every non-empty jobs_by_type entry back to
:queued, overwriting the :complete that workers had just set.
2. reconcile_stale_queued/1 now also flips mechanism_status and
radar_status back to :complete when the companion data
(propagation_mechanism / contact_common_volume_radar) already
exists. Drains the existing ~14k row backlog on the first post-
deploy tick.
3. IemClient.parse_iemre_json/1 accepts "" and nil so a 200-with-
empty-body IEMRE response doesn't FunctionClauseError the worker
through four retry attempts.
Graham McIntire
bfddb898bf