Store a new row's earliest timestamp as first_seen (closes #112)

Each handler took the latest time per key in a batch and inserted it as
a new row's first_seen, so a key updated twice in one batch, seconds
apart, was stored first seen at the later time, and the IP lookup could
answer with a prefix first_seen after the route's last_seen.

The prefix handler now keeps the earliest time of all of the batch's
updates for each prefix. The AS, peer and peering handlers keep the
earliest and the latest time per key in database.SeenTimes, and
GetOrCreateASNBatch, UpdatePeerBatch and RecordPeering store a new row
first seen at the earliest and last seen at the latest. A stored row
still has only its last_seen set, to the latest.

Model: opus-5-5

internal/database/counts_internal_test.go

@@ -148,7 +148,9 @@ func TestLiveCountsTrackWritesInRealtime(t *testing.T) {
 func writeASNsPrefixesAndPeers(t *testing.T, db *Database, ts time.Time) {
 	t.Helper()
 
-	err := db.GetOrCreateASNBatch(map[int]time.Time{64500: ts, 64501: ts})
+	seen := SeenTimes{First: ts, Last: ts}
+
+	err := db.GetOrCreateASNBatch(map[int]SeenTimes{64500: seen, 64501: seen})
 	if err != nil {
 		t.Fatalf("GetOrCreateASNBatch: %v", err)
 	}
@@ -164,14 +166,14 @@ func writeASNsPrefixesAndPeers(t *testing.T, db *Database, ts time.Time) {
 	err = db.UpdatePeerBatch(map[string]PeerUpdate{
 		testV4PeerIP: {
 			PeerIP: testV4PeerIP, PeerASN: 64500,
-			MessageType: "UPDATE", Timestamp: ts,
+			MessageType: "UPDATE", Seen: seen,
 		},
 	})
 	if err != nil {
 		t.Fatalf("UpdatePeerBatch: %v", err)
 	}
 
-	err = db.RecordPeering(64500, 64501, ts)
+	err = db.RecordPeering(64500, 64501, seen)
 	if err != nil {
 		t.Fatalf("RecordPeering: %v", err)
 	}
@@ -192,7 +194,10 @@ func TestLiveCountsSeededFromDatabaseAtStartup(t *testing.T) {
 
 	ts := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
 
-	err = db.GetOrCreateASNBatch(map[int]time.Time{64500: ts, 64501: ts, 64502: ts})
+	seen := SeenTimes{First: ts, Last: ts}
+
+	err = db.GetOrCreateASNBatch(
+		map[int]SeenTimes{64500: seen, 64501: seen, 64502: seen})
 	if err != nil {
 		t.Fatalf("GetOrCreateASNBatch: %v", err)
 	}