Add asn= and prefix= tags from a routewatch instance (closes #204)

With `SWW_ROUTEWATCH_URL` set, a new heuristic, `routewatch_tags`, adds `asn=<number>` and `prefix=<netblock>` to a request from what a routewatch instance answers about the client. New package `internal/routewatch` asks in the background, never on a request's path, and keeps answers 1 hour in memory. The request log gains `routewatch_tags_missing`, saying why a request has none. `prefix=` works in `SWW_TAG_DENY` and `SWW_TAG_LIMIT_PERCENT`.

Trips a reader: a client's first request has no routewatch tags, so `prefix=` refuses from its next request after the answer comes.
Judgement call: with `SWW_LOOKUP_SOURCE=off` and `SWW_ROUTEWATCH_URL` set, an `asn=` tag no longer stops the start.

Model: opus-5-5

internal/config/config_test.go

@@ -61,6 +61,7 @@ const (
 	reputationAction     = "SWW_REPUTATION_ACTION"
 	reputationCacheTTL   = "SWW_REPUTATION_CACHE_TTL"
 	reputationTimeout    = "SWW_REPUTATION_TIMEOUT"
+	routeWatchURL        = "SWW_ROUTEWATCH_URL"
 	crowdSecURL          = "SWW_CROWDSEC_LAPI_URL"
 	crowdSecKey          = "SWW_CROWDSEC_LAPI_KEY"
 	banResponse          = "SWW_BAN_RESPONSE"
@@ -1403,6 +1404,63 @@ func TestTagSettingsAsSet(t *testing.T) {
 	}
 }
 
+func TestRouteWatchURLAndPrefixTags(t *testing.T) {
+	t.Parallel()
+
+	const rw = "https://routewatch.example"
+
+	if cfg := fromEnvironment(t, environment{}); cfg.RouteWatchURL != "" {
+		t.Errorf("default %s is %q, want unset", routeWatchURL, cfg.RouteWatchURL)
+	}
+
+	cfg := fromEnvironment(t, environment{
+		routeWatchURL: rw, tagDeny: "prefix=2001:DB8::/32",
+		tagLimitPercent: "prefix=192.0.2.0/24:50",
+	})
+	wantDeny := []string{"prefix=2001:db8::/32"}
+
+	if cfg.RouteWatchURL != rw || !slices.Equal(cfg.TagDeny, wantDeny) ||
+		cfg.TagLimitPercent["prefix=192.0.2.0/24"] != 50 {
+		t.Errorf("gave %q, %v, %v", cfg.RouteWatchURL, cfg.TagDeny, cfg.TagLimitPercent)
+	}
+
+	// routewatch gives asn= tags too, so an asn= tag needs no lookups while it is set.
+	fromEnvironment(t, environment{
+		lookupSource: off, routeWatchURL: rw, tagDeny: "asn=64496",
+	})
+
+	for _, stopped := range []struct {
+		env  environment
+		name string
+	}{
+		{environment{routeWatchURL: "ftp://routewatch.example"}, routeWatchURL},
+		{environment{routeWatchURL: "routewatch.example"}, routeWatchURL},
+		{environment{tagDeny: "prefix=192.0.2.0/24"}, tagDeny},
+		{environment{routeWatchURL: rw, tagDeny: "prefix=nowhere"}, tagDeny},
+		{environment{routeWatchURL: rw, tagDeny: "prefix=192.0.2.5/24"}, tagDeny},
+		{
+			environment{routeWatchURL: rw, tagLimitPercent: "prefix=192.0.2.0/33:50"},
+			tagLimitPercent,
+		},
+		{environment{lookupSource: off, routeWatchURL: rw, tagDeny: "country=DE"}, tagDeny},
+		{environment{lookupSource: off, tagDeny: "asn=64496"}, tagDeny},
+	} {
+		_, err := config.FromEnvironment(stopped.env.lookupEnv)
+		if err == nil || !strings.HasPrefix(err.Error(), stopped.name+": ") {
+			t.Errorf("%v gave the error %v, want one naming %s", stopped.env, err, stopped.name)
+		}
+	}
+
+	badURL := environment{routeWatchURL: "ftp://routewatch.example"}
+	_, err := config.FromEnvironment(badURL.lookupEnv)
+
+	want := routeWatchURL + `: "ftp://routewatch.example" is not an http or https URL ` +
+		"without a user or a fragment, such as https://routewatch.example"
+	if err == nil || err.Error() != want {
+		t.Errorf("error %v, want %s", err, want)
+	}
+}
+
 func TestTagThatNoRequestCanGetIsToldApartFromOneItCan(t *testing.T) {
 	t.Parallel()
 
@@ -2503,6 +2561,7 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		reputationAction:     "limit:25",
 		reputationCacheTTL:   defaultReputationCacheTTL,
 		reputationTimeout:    "2s",
+		routeWatchURL:        "",
 		crowdSecURL:          "",
 		crowdSecKey:          "",
 		banResponse:          "403",