spaces: follow proposal 0016 PRs on HTTP-signature MACs and notification registration #27

Three open proposal-0016 PRs would change what zds has to do. None are merged, and no published SDK alpha carries them yet. Follow them, and adopt each once it merges and an alpha ships it.

• bluesky-social/proposals#141: MACs as HTTP Message Signatures (approved, not merged)
  • MACs move to RFC 9421 hmac-sha256: requested with Accept-Signature, returned in Signature-Input / Signature.
  • The signed content moves into new Atproto-Space-Commit and Atproto-Space-Record headers.
  • Atproto-Space-Want-Mac and Atproto-Space-Mac go away.
  • Space credential signatures use the label sig, with the atproto-space-credential tag still checked.
  • For zds: rework MAC request and response handling. Serve both forms until syncers on the November 5 alpha have moved, as with the lexicon renames.
• bluesky-social/proposals#138: register for notifications in getSpaceCredential (open)
  • registerNotify and unregisterNotify are replaced by a notifyService field on the credential request.
  • A registration lasts until that credential expires, plus one more notification.
  • A permanent HTTP error from the syncer unregisters it, and revoking the credential drops it.
  • Lapsed registrations still receive the space-deletion notice.
• bluesky-social/proposals#140: webhook notifications (open, built on #138)
  • notifyService becomes notify: {endpoint, token}.
  • The space host authenticates to syncers with that opaque token instead of service auth.
  • Repo-host-to-space-host notifyWrite is unchanged.

Not tracked here:

• No change for zds:
  • #136 (lex refactor): zds 0.7.7 already serves it, as atproto#5800.
  • #137 (draft): the name → title rename in space type declarations, which zds doesn't read.
  • #139 / #130: guidance for apps, not hosts.
• Stalled: #91 (0015 account transfer), no movement since May.