Determine staff members based on flag

This replaces the old system based on email addresses, the aim is to make it more generic and thus appropriate for OSS/self-hosting

See: https://3.basecamp.com/2914079/buckets/37331921/todos/9302705265#__recording_9320051455

db/migrate/20251125130010_add_a_staff_flag_to_identities.rb

@@ -0,0 +1,5 @@
+class AddAStaffFlagToIdentities < ActiveRecord::Migration[8.2]
+  def change
+    add_column :identities, :staff, :boolean, null: false, default: false
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.2].define(version: 2025_11_25_110629) do
+ActiveRecord::Schema[8.2].define(version: 2025_11_25_130010) do
   create_table "accesses", id: :uuid, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "accessed_at"
     t.uuid "account_id", null: false
@@ -300,6 +300,7 @@ ActiveRecord::Schema[8.2].define(version: 2025_11_25_110629) do
   create_table "identities", id: :uuid, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.string "email_address", null: false
+    t.boolean "staff", default: false, null: false
     t.datetime "updated_at", null: false
     t.index ["email_address"], name: "index_identities_on_email_address", unique: true
   end