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

PrivaPub/Infrastructure/Backup/ProtectiveMerge.cs

@@ -168,6 +168,9 @@ namespace PrivaPub.Infrastructure.Backup
 					.Set("CredentialsChangedAt", now), cancellationToken: token);
 			foreach (var collection in new[] { "openiddict.tokens", "openiddict.authorizations" })
 				report.SessionsEnded += (int)(await database.GetCollection<BsonDocument>(collection).DeleteManyAsync(FilterDefinition<BsonDocument>.Empty, token)).DeletedCount;
+			// and the Web Push subscriptions made with those tokens (R9; never in a backup): clients subscribe again once signed
+			// in. The VAPID key (PushKeys, never in a backup either) stays the live one.
+			await database.GetCollection<BsonDocument>("PushSubscription").DeleteManyAsync(FilterDefinition<BsonDocument>.Empty, token);
 
 			// what the backup remembered of servers being down is long past
 			await database.GetCollection<BsonDocument>("RemoteInstance").UpdateManyAsync(FilterDefinition<BsonDocument>.Empty,