pkarr lookup service (P7.2)

This Worker is a self-hosted pkarr relay + DNS-over-HTTPS endpoint for the
id system. It stores signed pkarr packets and serves them back over HTTP:

  PUT /pkarr/<z32-public-key>   publish a signed packet
  GET /pkarr/<z32-public-key>   fetch the current packet
  POST /dns-query               DoH TXT lookups for _iroh.<key>.<origin>

It is lookup only: not a Mainline DHT node, not a general DNS server, not
a traffic relay, and not a replacement for the default n0 pkarr service.
Records are signed by the endpoint key and expire after a retention
window without refresh.