Add small Spamfilter service wrapping Nyckel API

app/services/spamfilter_service.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+class SpamfilterService
+  class NyckelError < StandardError; end
+  class NotAuthorizedError < NyckelError; end
+  Result = Struct.new(:labelName, :confidence) do
+    def spam?
+      labelName == 'Spam'
+    end
+
+    def ham?
+      labelName == 'Ham'
+    end
+  end
+
+  NYCKEL_URL = 'https://www.nyckel.com'
+  NYCKEL_FUNCTION = ENV.fetch('NYCKEL_FUNCTION', nil)
+
+  def self.call(account_age_hours:, content:)
+    Retriable.retriable(
+      on_retry: -> { refresh_token! },
+      on: [NotAuthorizedError]
+    ) do
+      response = HTTP.auth("Bearer #{access_token}").post(
+        "#{NYCKEL_URL}/v1/functions/#{NYCKEL_FUNCTION}/invoke",
+        json: { data: { account_age_hours:, content: } }
+      )
+      body = JSON.parse(response.body.to_s)
+
+      raise NotAuthorizedError if response.code == 401
+      raise NyckelError, body.message if response.code != 200
+
+      Result.new(body['labelName'], body['confidence'])
+    end
+  end
+
+  def self.access_token
+    refresh_token! if @access_token.nil? || @access_token.expired?
+
+    @access_token.token
+  end
+
+  def self.refresh_token!
+    @access_token = oauth2.get_token
+  end
+
+  def self.oauth2
+    OAuth2::Client.new(
+      ENV.fetch('NYCKEL_CLIENT_ID', nil),
+      ENV.fetch('NYCKEL_CLIENT_SECRET', nil),
+      site: NYCKEL_URL,
+      token_url: '/connect/token'
+    ).client_credentials
+  end
+end