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/Models/PushKeys.cs

@@ -0,0 +1,18 @@
+using MongoDB.Entities;
+
+namespace PrivaPub.Models
+{
+	// The server's VAPID key pair (RFC 8292, completion program R9): one P-256 key, made the first time something needs it
+	// (Infrastructure/Push/Vapid) and kept here, one document with the fixed id `vapid`. Every push carries a JWT it signs,
+	// and the push services tie each subscription to its public key, which the instance API and app registrations
+	// advertise. Never in a backup and never dropped by a restore (ServerBackup.Excluded): a restored key would break every
+	// subscription a client made since.
+	public class PushKeys : Entity
+	{
+		public const string Name = "vapid";
+
+		public string PublicKey { get; set; }//the uncompressed point (0x04 || X || Y), base64url without padding
+		public string PrivateKey { get; set; }//the private scalar D, base64url without padding
+		public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+	}
+}