Resolve PTR names over DNS-over-HTTPS and cache them for the record TTL (closes #238)

`ptr` keeps its field and shape. It now comes from Google, Cloudflare, AdGuard and Quad9 over RFC 8484, tried in turn within 2 seconds, in place of the host's resolver with no cache. Answers stay in memory and in a new `cache.db` for the record's TTL, at most 250,000 addresses. A reserved address is not sent to any resolver and has an empty `ptr`.

Judgement call: each resolver gets an equal share of the time left, so a hung one does not use up the limit.
Judgement call: the negative TTL is the smaller of the SOA minimum and the SOA's own TTL.
Deviation: the cache is SQLite, by sneak's words, not the JSON file the standing rule asks for.

Model: opus-5-5

internal/server/middleware_internal_test.go

@@ -115,7 +115,7 @@ func TestJSONAnswersPutNoUnitInAString(t *testing.T) {
 		`^([0-9]+(\.[0-9]+)? ?(ns|µs|ms|s|m|h|B|kB|MB|GB|TB))+$`)
 
 	s := newServerWithTestRoutes(t)
-	s.resolver = &fakeResolver{}
+	useFakeResolver(s, &fakeResolver{})
 	s.SetASNFetcher(fixedIntervalASNFetcher{interval: time.Second})
 
 	paths := []string{
@@ -179,7 +179,7 @@ func TestJSONAnswersCarryMeta(t *testing.T) {
 	)
 
 	s := newServerWithTestRoutes(t)
-	s.resolver = &fakeResolver{}
+	useFakeResolver(s, &fakeResolver{})
 	s.streamer.GetMetricsTracker().SetConnected(true)
 	s.router.Get(writesNothingPath, JSONValidationMiddleware(http.HandlerFunc(
 		func(http.ResponseWriter, *http.Request) {})).ServeHTTP)
@@ -188,7 +188,7 @@ func TestJSONAnswersCarryMeta(t *testing.T) {
 
 	failing := New(failingDB{}, newTestTable(),
 		streamer.New(logger.New(), metrics.New()), logger.New(), &config.Config{})
-	failing.resolver = &fakeResolver{}
+	useFakeResolver(failing, &fakeResolver{})
 
 	tests := []struct {
 		method   string
@@ -521,7 +521,7 @@ func newServerWithConfig(
 
 	s := New(db, newTestTable(), streamer.New(logger.New(), metrics.New()),
 		&logger.Logger{Logger: slog.New(slog.NewJSONHandler(requestLog, nil))}, cfg)
-	s.resolver = &fakeResolver{}
+	useFakeResolver(s, &fakeResolver{})
 
 	return s
 }