Remove foreign key constraint from devices to sessions

Devices should persist independently of sessions - when a session is
deleted, the device registration should remain valid.

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

db/migrate/20260114203313_create_action_push_native_devices.rb

@@ -5,7 +5,7 @@ class CreateActionPushNativeDevices < ActiveRecord::Migration[8.0]
       t.string :platform, null: false
       t.string :token, null: false
       t.belongs_to :owner, polymorphic: true, type: :uuid, index: false
-      t.belongs_to :session, type: :uuid, foreign_key: true
+      t.belongs_to :session, type: :uuid
 
       t.timestamps
     end

db/schema.rb

@@ -853,6 +853,4 @@ ActiveRecord::Schema[8.2].define(version: 2026_02_18_120000) do
     t.index ["account_id"], name: "index_webhooks_on_account_id"
     t.index ["board_id", "subscribed_actions"], name: "index_webhooks_on_board_id_and_subscribed_actions", length: { subscribed_actions: 255 }
   end
-
-  add_foreign_key "action_push_native_devices", "sessions"
 end