perf(db): index packets(last_heard_at)

The packet list sorts by last_heard_at, so every page was a seq scan
plus sort over all packets. Cheap to maintain now that presence writes
are coalesced.

db/migrations/008_packets_last_heard_index.sql

@@ -0,0 +1,8 @@
+-- 008_packets_last_heard_index.sql
+--
+-- The packet list orders by last_heard_at but there was no index on it, so
+-- every page seq-scanned and sorted all packets (~2.8s per page on a 1.5M
+-- row table; 31ms with the index). Affordable to maintain now that presence
+-- coalescing keeps this column from being rewritten on every observation.
+
+CREATE INDEX idx_packets_last_heard ON packets(last_heard_at DESC);