Allow long user agent strings

Copied this from BC and HEY which also use a string with a 4096 character length

See: https://app.fizzy.do/5986089/cards/3066

db/migrate/20251125110629_increase_user_agent_length.rb

@@ -0,0 +1,6 @@
+class IncreaseUserAgentLength < ActiveRecord::Migration[8.2]
+  def change
+    change_column :sessions, :user_agent, :string, limit: 4096
+    change_column :push_subscriptions, :user_agent, :string, limit: 4096
+  end
+end