feat: add configurable Postal logger level defaulting to INFO (#3626)

lib/postal/config.rb

@@ -83,6 +83,7 @@ module Postal
     def logger
       @logger ||= begin
         k = Klogger.new(nil, destination: Config.logging.enabled? ? $stdout : "/dev/null", highlight: Config.logging.highlighting_enabled?)
+        k.level = Config.logging.level
         k.add_destination(graylog_logging_destination) if Config.gelf.host.present?
         k
       end

lib/postal/config_schema.rb

@@ -227,6 +227,19 @@ module Postal
         default true
       end
 
+      string :level do
+        description "The minimum log level for the Postal logger (debug, info, warn, error, fatal)"
+        default "INFO"
+        transform do |value|
+          normalized = value.to_s.strip.downcase
+          normalized = "info" if normalized.empty?
+          unless %w[debug info warn error fatal].include?(normalized)
+            raise ArgumentError, "logging.level must be one of: debug, info, warn, error, fatal (got #{value.inspect})"
+          end
+          normalized
+        end
+      end
+
       boolean :highlighting_enabled do
         description "Enable highlighting of log lines"
         default false