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

`internal/ptr` asks Google, Cloudflare, AdGuard and Quad9 in turn, 2 seconds in all per address. It keeps answers in memory and writes them through to `cache.db`, opened by the new `internal/cachedb`. A reserved address is not sent to the resolvers.

Judgement calls:
- Each resolver gets an equal share of the time left, so a hung resolver does not use up the 2 seconds.
- The negative TTL is the smaller of the SOA minimum and the SOA's own TTL (RFC 2308).
- Names that are not host names are dropped, as `net.Resolver.LookupAddr` does.

Model: opus-5-5

internal/server/middleware_internal_test.go

@@ -114,7 +114,7 @@ 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{
@@ -174,7 +174,7 @@ 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)
@@ -239,7 +239,7 @@ func TestJSONAnswersCarryMeta(t *testing.T) {
 	// handlers of the other routes 500.
 	failing := New(failingDB{}, newTestTable(),
 		streamer.New(logger.New(), metrics.New()), logger.New(), &config.Config{})
-	failing.resolver = &fakeResolver{}
+	useFakeResolver(failing, &fakeResolver{})
 
 	checkAnswerCarriesMeta(t, failing, httptest.NewRequestWithContext(
 		t.Context(), http.MethodGet, healthCheckPath, nil),
@@ -514,7 +514,7 @@ 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
 }