Move category and high_priority to payload classes

Use polymorphism instead of case statements in Native push target:
- DefaultPayload#category returns "default", #high_priority? returns false
- EventPayload#category returns "assignment"/"comment"/"card" based on action
- MentionPayload#category returns "mention", #high_priority? returns true

This simplifies the Native push target by delegating source-specific
logic to the appropriate payload classes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/models/notification/default_payload.rb

@@ -23,6 +23,14 @@ class Notification::DefaultPayload
     notifications_url
   end
 
+  def category
+    "default"
+  end
+
+  def high_priority?
+    false
+  end
+
   private
     def card_url(card)
       Rails.application.routes.url_helpers.card_url(card, **url_options)

app/models/notification/event_payload.rb

@@ -36,6 +36,18 @@ class Notification::EventPayload < Notification::DefaultPayload
     end
   end
 
+  def category
+    case event.action
+    when "card_assigned" then "assignment"
+    when "comment_created" then "comment"
+    else "card"
+    end
+  end
+
+  def high_priority?
+    event.action.card_assigned?
+  end
+
   private
     def event
       notification.source

app/models/notification/mention_payload.rb

@@ -13,6 +13,14 @@ class Notification::MentionPayload < Notification::DefaultPayload
     card_url(card)
   end
 
+  def category
+    "mention"
+  end
+
+  def high_priority?
+    true
+  end
+
   private
     def mention
       notification.source