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/discriminator/heuristics.go

@@ -44,6 +44,7 @@ const (
 func beforeLookup() []heuristic {
 	return []heuristic{
 		tagLists{},
+		routeWatchTags{},
 		denyNets{},
 		activeBan{},
 	}