Never move last seen back or first seen forward (closes #162)

Some RIS collectors send updates days old while they catch up on a
backlog. Every update to a stored AS, peering or peer set last_seen to
the update's time, even an older one, and never lowered first_seen. An
AS page could then list a peer first seen after it was last seen.

The update statements for asns, peerings and bgp_peers now keep the
earlier of the stored and new first_seen and the later of the stored
and new last_seen, with MIN and MAX in the SQL. The batch and the
single-row writes share each statement. GetOrCreateASN returns the
times the row now holds. Route times are unchanged.

Model: opus-5-5

TODO.md

@@ -33,6 +33,10 @@ tested.
 
 # Completed Steps
 
+- 2026-10-26: a write to a stored AS, peering or peer keeps the earlier of the
+  two first-seen times and the later of the two last-seen times. An old update
+  from a collector catching up on a backlog no longer moves last seen back
+  (closes #162)
 - 2026-10-26: the status page shows "Route Churn/min", the number of prefix
   announcements and withdrawals, from all peers, in the last full clock minute.
   `/api/v1/stats` reports it as `route_churn_per_min` in `stream`, which