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

tools/pasture/Caddyfile

@@ -207,6 +207,11 @@ lemmy19.test {
 	reverse_proxy pasture-lemmy19:8536
 }
 
+pushsink.test {
+	tls internal
+	reverse_proxy pasture-pushsink:8080
+}
+
 flood1.test, flood2.test, flood3.test, flood4.test, flood5.test, flood6.test, flood7.test, flood8.test, flood9.test, flood10.test, flood11.test, flood12.test, flood13.test, flood14.test, flood15.test, flood16.test, flood17.test, flood18.test, flood19.test, flood20.test {
 	tls internal
 	reverse_proxy pasture-flood:8080