Security fixes: CSV injection, policy body output, invite hashing, slot datetimes

Four fixes from a security review pass:

- Neutralise CSV formula injection in the payments export: fields with a
  leading =, +, -, @, tab, or CR (e.g. a hostile student display name) are
  apostrophe-prefixed in PaymentReport::csvLine() so they open as text in
  Excel/Google Sheets. Fixes #39.
- Sanitise policy bodies with wp_kses_post at output in
  PolicyEndpoint::index() (the booking JS renders that HTML raw), so a
  future write path that forgets kses can never become stored XSS.
  Fixes #40.
- Store invite tokens hashed (SHA-256) at rest: a database leak can no
  longer redeem pending invites. The registration link is shown once, at
  creation; the pending list shows email/invited date; lookups hash the
  submitted token. Existing plaintext pending invites must be re-issued.
  Fixes #41.
- Validate availability slot datetimes on both creation paths (REST and
  admin form) via AvailabilitySlot::normalizeDateTime(): canonical and
  datetime-local forms normalise to Y-m-d H:i:s, garbage and end <= start
  are rejected (REST 400) instead of reaching the DATETIME column or
  throwing inside the weekly-series date arithmetic. Fixes #42.

composer test (204 tests, 594 assertions), PHPStan L6, and PHPCS all green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/frontend/register-page.php

@@ -7,6 +7,7 @@ if (! defined('ABSPATH')) {
 
 /**
  * @var \Unsupervised\Schedular\Auth\Invite|null $invite
+ * @var string $token Raw invite token from the request (only its hash is stored).
  * @var bool   $canRegister
  * @var bool   $success
  * @var string $error
@@ -25,7 +26,7 @@ if (! defined('ABSPATH')) {
 
         <form method="post" action="">
             <?php wp_nonce_field('us_student_register'); ?>
-            <input type="hidden" name="us_invite" value="<?php echo esc_attr($invite->token); ?>">
+            <input type="hidden" name="us_invite" value="<?php echo esc_attr($token); ?>">
 
             <p>
                 <label for="us-reg-email"><?php esc_html_e('Email', 'unsupervised-schedular'); ?></label>