GET /observers/{id}/activity returns bucketed observation counts, LoRa
airtime, SNR/RSSI aggregates and a payload-type breakdown for one
observer. Airtime is computed at ingest from the raw frame length and
stored on the observation; intervals of 1h and up are served from an
hourly matview so no request path joins back to packets.
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