Lookups turned every error into a 404, and region filters into a 400, so a
transient database failure looked final to clients that don't retry 4xx.
Missing rows stay 404/400; anything else is a 500. Empty /regions and
/iatas lists are [] rather than null.
Refresh last_heard on duplicate observations too, capped at hourly, so
steady traffic can't age a channel out of the filter while its packets
stay retained (dedup key has no heard_at). Guard the seed join against
the docker /dev/shm cap like the trace one, and drop the unused
last_heard index column so the upserts stay HOT.
The IATA filter ran a correlated EXISTS over packets/observations with
ILIKE, which skipped the iata index and took ~7s live. Keep a small
channel_iatas table at ingest (like node_iatas) and filter against it.
Also honor the iatas= param so multi-site regions stop getting the
global list. Filter now ages out with packet retention rather than
matching any retained packet.
MrAlders0n