Add "data export" feature

- Adds a button in Account Settings where you can request a ZIP export of your
  Fizzy data
- Export files are created in the background. When ready, a link to
  download them is sent to the requester.
- Exports expire after 24 hours. And are limited to 10 per day.

db/migrate/20251201100607_create_account_exports.rb

@@ -0,0 +1,14 @@
+class CreateAccountExports < ActiveRecord::Migration[8.2]
+  def change
+    create_table :account_exports, id: :uuid do |t|
+      t.uuid :account_id, null: false
+      t.uuid :user_id, null: false
+      t.string :status, default: "pending", null: false
+      t.datetime :completed_at
+      t.timestamps
+
+      t.index :account_id
+      t.index :user_id
+    end
+  end
+end