refactor(referral): prune dead code and formalize ops list header

- remove unused AuthReader::invitation_mode and ReferralAttribution
  helpers (get_or_create_code / validate_ref); contract test now
  asserts the canonical invit_code_s reader in CreditsRuntime
- drop legacy aksrt_invite table from uninstall cleanup list
- track referral code creation time (aksrt_referral_code_created)
  and surface it in the referral ops REST list
- formalize ops list headers (归属用户/推广码/启用状态/推广用户数/
  有效付费用户数/创建时间) and right-align count columns

tests/widget-system/auth-source-contract.test.cjs

@@ -162,7 +162,6 @@ test('shared renderer contracts every configured authentication state', () => {
   const shared = read('templates/parts/auth-content.php');
   const modal = read('templates/parts/modal-auth.php');
   const client = read('assets/js/auth.js');
-  const settings = read('src/Settings/AuthReader.php');
 
   assert.match(shared, /data-auth-password-fields/);
   assert.match(shared, /data-auth-code-fields/);
@@ -193,8 +192,8 @@ test('shared renderer contracts every configured authentication state', () => {
   assert.match(client, /form\.checkValidity\(\)[\s\S]*?field\.validationMessage/);
   assert.match(client, /window\.location\.assign\(authPageUrl/);
   assert.doesNotMatch(client + shared, /showMessage|data-auth-message|sign-message/);
-  assert.match(settings, /invitation_mode/);
-  assert.match(settings, /\['close', 'open', 'must'\]/);
+  assert.match(read('src/Settings/CreditsRuntime.php'), /invit_code_s/);
+  assert.match(read('src/Settings/CreditsRuntime.php'), /\['close', 'open', 'must'\]/);
 });
 
 test('OAuth frontend is accessible, redirect-aware, and removes retired providers', () => {