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

docs/request-log.md

@@ -173,6 +173,12 @@ Every one of these lines has:
     - `abuseipdb`.
 - `tags`: the request's tags, sorted, left out when it has none. Each tag and
   where it comes from is under "Request tags" in [`SPEC.md`](../SPEC.md).
+- `routewatch_tags_missing`: why the request got no tags from routewatch,
+  present only while `SWW_ROUTEWATCH_URL` is set:
+    - `no_answer_yet`: routewatch has not answered about this client yet;
+    - `request_failed`: a query to routewatch failed within the last minute, and
+      the process log says why;
+    - `no_route`: routewatch knows no live route for the address.
 - `offence`: `limit`, for a request that broke a limit.
 - `ban_expires`: when the client's ban ends, for a request that made one or was
   refused under one: a time, or `permanent`.

docs/settings.md

@@ -145,8 +145,8 @@ cut off; the client is banned.
   the start while a setting that needs a lookup is set:
     - `SWW_EXCLUSIVELY_ALLOWED_COUNTRIES`;
     - `SWW_ADD_LOOKUP_HEADERS`;
-    - `SWW_TAG_DENY` or `SWW_TAG_LIMIT_PERCENT` naming an `asn=` or `country=`
-      tag;
+    - `SWW_TAG_DENY` or `SWW_TAG_LIMIT_PERCENT` naming a `country=` tag, or an
+      `asn=` tag while `SWW_ROUTEWATCH_URL` is unset;
     - the two byte percentage lists and `SWW_ASN_LIMIT_PERCENT_URL`;
     - `SWW_UNKNOWN_LIMIT_PERCENT` below `100`;
     - an anomaly threshold per AS number.
@@ -186,11 +186,20 @@ Every request gets tags from what `discriminator` knows of its client. See
   `exploitsignature`: a request gets that tag after the rate limits have counted
   it.
 
+- `SWW_ROUTEWATCH_URL` (default unset): the `http` or `https` URL of a
+  routewatch instance. [routewatch](https://git.eeqj.de/sneak/routewatch) is a
+  service that gives an address's announced netblock and origin AS.
+  `discriminator` asks it about each client, in the background, and adds the
+  tags `asn=<number>` and `prefix=<netblock>` from its answer. While it is
+  unset, nothing asks routewatch and no request gets a `prefix=` tag.
+
 `SWW_TAG_DENY` and `SWW_TAG_LIMIT_PERCENT` accept tags in either case. A tag
 that no request can get stops the start. A request can get only these:
 
 - a name in `SWW_TAG_LISTS`;
 - an `asn=` or `country=` tag;
+- a `prefix=` tag with a netblock, while `SWW_ROUTEWATCH_URL` is set. It matches
+  only that exact announced netblock, such as `prefix=192.0.2.0/24`;
 - a `dnsbl=` tag of a zone in `SWW_DNSBL_ZONES`, in lower case, with a key
   masked as the `reputation` field shows it;
 - `abuseipdb`, while `SWW_ABUSEIPDB_KEY` is set;