Implement Passkey registration

- Create Identity credentials
- Add management UI and registration flow

db/migrate/20260213154740_create_identity_credentials.rb

@@ -0,0 +1,17 @@
+class CreateIdentityCredentials < ActiveRecord::Migration[8.2]
+  def change
+    create_table :identity_credentials, id: :uuid do |t|
+      t.uuid :identity_id, null: false
+      t.string :credential_id, null: false
+      t.binary :public_key, null: false
+      t.integer :sign_count, null: false, default: 0
+      t.string :name
+      t.text :transports
+
+      t.timestamps
+
+      t.index :identity_id
+      t.index :credential_id, unique: true
+    end
+  end
+end