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/requestlog/requestlog.go

@@ -182,6 +182,10 @@ type Line struct {
 	Reputation []string `json:"reputation,omitempty"`
 	// Tags are the request's tags, sorted.
 	Tags []string `json:"tags,omitempty"`
+	// RouteWatchTagsMissing, while SWW_ROUTEWATCH_URL is set, says why the
+	// request got no tags from routewatch: no_answer_yet, request_failed or
+	// no_route.
+	RouteWatchTagsMissing string `json:"routewatch_tags_missing,omitempty"`
 	// Offence is the offence the request was held as, OffenceLimit.
 	Offence string `json:"offence,omitempty"`
 	// BanExpires is when the ban the request made, or was refused under,