R9 Web Push, then the honest version: VAPID key, push subscriptions, encrypted pushes, 4.5.0 advertised

The server's VAPID key (PushKeys, made on first need, in Mongo) is the instance API's vapid.public_key and every app's
vapid_key. /api/v1/push/subscription (POST, GET, PUT, DELETE) keeps one subscription per access token, so per persona;
every notification kept (never a filtered one) is queued as a SendPush job for the subscriptions whose alert and policy
admit it, and sent through the `push` client (the safe handler, https only) encrypted as RFC 8291 aes128gcm or the
aesgcm draft with an ES256 VAPID JWT. 404/410 delete the subscription, 429/5xx/timeouts retry, other answers drop the
push. Subscriptions end with their tokens (revocation, sessions ended, suspension, deletion, a restore); backups hold
neither keys nor subscriptions. The walk of every Mastodon 4.3-4.5 client route (docs/MASTODON-API.md) added
conversations/:id/unread, domain_blocks/preview, v2 notifications clear and PUT policy, /oauth/userinfo and stubs, so both
instance APIs now say 4.5.0 (compatible; PrivaPub <release>). The pasture gets a push sink (pushsink.test) and
scenarios/push.sh in batch A.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WpVCoDeM1qUTRp4fXQdfof

FEDERATION.md

@@ -456,6 +456,14 @@ PrivaPub keeps statistics about servers, never about accounts (see `/stargazing`
   - **Opting out:** in robots.txt, the crawler obeys the group `PrivaPub-Stargazer`, then `PrivaPub`, then `*`. A
     robots.txt that answers with a server error or times out also keeps it out, as do this server's domain blocks.
 
+## Web Push
+
+Not federation, but traffic an operator may see: when a client signed in as a persona subscribes to Web Push (Mastodon's
+`/api/v1/push/subscription`), PrivaPub POSTs its notifications to the push service the client named, encrypted to the
+client (RFC 8291 `aes128gcm`, or the older `aesgcm` draft Mastodon still sends) and carrying a VAPID JWT (RFC 8292) signed
+by the server's own P-256 key, the one `/api/v2/instance` gives as `configuration.vapid.public_key`. No HTTP signature
+and no ActivityPub; https endpoints on public addresses only, no redirects. The push service learns when, never what.
+
 ## Local-only posts
 
 Posts with a location (shown to nearby users of this server) never leave the server, in any form.