Keep a trailing AS set out of the origin and the peerings (closes #75)

A path that ends in an AS set of several members, such as
[64496, 64497, [64498, 64499]], now has no origin: its route is stored
with origin 0 (none), and no member of the set is in a peering. A set of
one member counts as that AS. An announcement with an empty path is
still not stored.

ASPath keeps such a set apart from the sequence and gives the origin.
The prefix and AS handlers take the origin from it; the peering handler
uses the sequence only. The stored AS path still lists the set's members
at the end.

The prefix and prefix length pages show origin 0 as no known origin, the
IP lookup asks no WHOIS lookup for it, and the README says what it means.

Model: opus-5-5

internal/database/database.go

@@ -2141,11 +2141,12 @@ func (d *Database) GetIPInfoContext(
 		info.FirstSeen = prefixFirstSeen.Time
 	}
 
-	// Check if WHOIS data needs refresh (never fetched or older than 30 days)
+	// Check if WHOIS data needs refresh (never fetched or older than 30 days).
+	// A route with origin 0 has no known origin, so no AS to look up.
 	const staleThreshold = 30 * 24 * time.Hour
 
-	info.NeedsWHOISRefresh = !whoisUpdatedAt.Valid ||
-		time.Since(whoisUpdatedAt.Time) > staleThreshold
+	info.NeedsWHOISRefresh = info.ASN != 0 && (!whoisUpdatedAt.Valid ||
+		time.Since(whoisUpdatedAt.Time) > staleThreshold)
 
 	return info, nil
 }