perf(routes): index known_routes(last_seen)

The routes list orders by last_seen with no index, so every page
seq-scanned and sorted ~150k rows. Same fix as the packets list got
in 008.

db/migrations/014_known_routes_last_seen_index.sql

@@ -0,0 +1,3 @@
+-- Routes list orders by last_seen; index it (was seq-scanning ~150k rows per page).
+
+CREATE INDEX idx_known_routes_last_seen ON known_routes(last_seen DESC);