feat(federation): add approval_token columns to peer schema

Nullable text columns on federation_peers and peer_approval_requests.
Existing rows degrade gracefully (NULL token) per spec §5; the verification
logic landing in subsequent commits routes legacy null-token state through
the existing autoAccept gate.

packages/server/drizzle/0002_peer_approval_token.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `federation_peers` ADD `approval_token` text;--> statement-breakpoint
+ALTER TABLE `peer_approval_requests` ADD `approval_token` text;