Retry CC feature - #17

🟠 `retry_charge_card()` writes `App::variables()->request['ot_cc']` and `['retry_cc']` at lines 765-766 but never clears them before returning — no `unset()`, no `try/finally`. In a long-running PHP process that iterates over customers (billing cron, queue worker), customer A's retry poisons subsequent iterations: customer B's `charge_card()` reads the stale `ot_cc` and, because `parse_ccs()` uses sequential 0/1/2 int keys, is highly likely to collide with B's `$ccs` — silently charging B's **wr

RETRY_CC at src/cc.inc.php:573 is referenced bare (`RETRY_CC == 1`) without a `defined()` guard, but the constant is declared nowhere in the plugin (repo-wide grep for RETRY_CC returns only this line and the two `retry_cc` request-param uses). On a host that hasn't externally defined RETRY_CC, PHP 8.0+ throws a fatal Error and the entire decline branch 500s before the retry logic, the paypal fallback, or the decline email run; on PHP 7.x it emits E_WARNING and `'RETRY_CC' == 1` evaluates to fals

This PR deletes `make_cc_decline()` and `email_cc_decline()` from `src/cc.inc.php` but leaves three stale references: `src/Plugin.php:67-68` still registers them as lazy-loadable requirements, `tests/CcFunctionsTest.php:255-256` asserts their `function make_cc_decline(`/`function email_cc_decline(` strings are present (so `testAllExpectedFunctionsDeclared()` now fails deterministically), and `CLAUDE.md:33` still documents them as core CC functions. Fix by either dropping the Plugin.php registrat

The decline branch builds the payment-failed email body into $email via $smarty->fetch('email/client/payment_failed.tpl') at src/cc.inc.php:567, but the `(new \MyAdmin\Mail())->multiMail(...)` call that actually sent it was deleted in this PR — the variable is computed and silently discarded. Customers now receive no decline notification in any path (retry succeeds, retry fails, or the else-branch paypal downgrade), which is a regression from the prior unconditional send. Fix by restoring the mu