Refactor Nostr settings/connect

* Use NIP-42 auth event instead of short text note
* Verify event ID and signature using the nostr gem instead of custom code

app/services/nostr_manager/verify_auth.rb

@@ -0,0 +1,17 @@
+module NostrManager
+  class VerifyAuth < NostrManagerService
+    def initialize(event:, challenge:)
+      @event = event
+      @challenge_expected = challenge
+      @site_expected = Setting.accounts_domain
+    end
+
+    def call
+      site_given      = @event.tags.find{|t| t[0] == "site"}[1]
+      challenge_given = @event.tags.find{|t| t[0] == "challenge"}[1]
+
+      site_given == @site_expected &&
+      challenge_given == @challenge_expected
+    end
+  end
+end