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/server.go

@@ -52,10 +52,14 @@ type IPLists interface {
 	ShortestInterval() time.Duration
 }
 
-// dnsResolver looks up the addresses of host names and the names of addresses
-// in DNS, as *net.Resolver does.
+// dnsResolver looks up the addresses of host names in DNS, as *net.Resolver
+// does.
 type dnsResolver interface {
 	LookupHost(ctx context.Context, host string) ([]string, error)
+}
+
+// ptrResolver looks up the PTR names of addresses, as *ptr.Cache does.
+type ptrResolver interface {
 	LookupAddr(ctx context.Context, addr string) ([]string, error)
 }
 
@@ -75,10 +79,13 @@ type Server struct {
 	// RealIPMiddleware believes
 	trustedProxies []netip.Prefix
 
-	// resolver makes the IP lookup's DNS lookups: net.DefaultResolver, or a
+	// resolver looks up the IP lookup's host names: net.DefaultResolver, or a
 	// fake in tests
 	resolver dnsResolver
 
+	// ptrResolver looks up the IP lookup's PTR names. SetPTRResolver sets it.
+	ptrResolver ptrResolver
+
 	// cards is the copy of the home page's cards that the pages serve, nil
 	// until the server first works them out
 	cards atomic.Pointer[homeCards]
@@ -196,6 +203,12 @@ func (s *Server) Stop(ctx context.Context) error {
 	return nil
 }
 
+// SetPTRResolver sets what looks up the IP lookup's PTR names. Call it before
+// Start: the handlers read it without a lock.
+func (s *Server) SetPTRResolver(resolver ptrResolver) {
+	s.ptrResolver = resolver
+}
+
 // SetASNFetcher sets the ASN WHOIS fetcher for on-demand lookups. Call it
 // before Start: the handlers read the fetcher without a lock.
 func (s *Server) SetASNFetcher(fetcher ASNFetcher) {