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. The cache holds at most 250,000 addresses.

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

TODO.md

@@ -53,6 +53,9 @@ problem the run finds.
 
 # Completed Steps
 
+- 2026-11-10: the IP lookup finds PTR names over DNS-over-HTTPS and caches them
+  for the record's TTL, in memory and in `cache.db` (closes
+  https://git.eeqj.de/sneak/routewatch/issues/238).
 - 2026-11-10: the tests no longer fail when a busy host stalls for several
   seconds. Most such waits are now 15 seconds. The test of the stream's idle
   timeout runs on a fake clock (closes #240)