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 <[email protected]>

src/Auth/RegistrationController.php

@@ -17,8 +17,9 @@ class RegistrationController {
 			wp_die( esc_html__( 'You do not have permission to manage invites.', 'unsupervised-schedular' ) );
 		}
 
+		$newInviteUrl = '';
 		if ( isset( $_POST['usc_action'] ) && check_admin_referer( 'usc_invite_action' ) ) {
-			$this->handleFormAction();
+			$newInviteUrl = $this->handleFormAction();
 		}
 
 		$pendingInvites      = $this->invites->findPending();
@@ -28,7 +29,12 @@ class RegistrationController {
 		include USC_PLUGIN_DIR . 'templates/admin/invites.php';
 	}
 
-	private function handleFormAction(): void {
+	/**
+	 * Handle a posted admin action. Returns the registration link for a freshly
+	 * created invite — the only time it can be shown, since just the token's hash
+	 * is stored — or an empty string for every other action.
+	 */
+	private function handleFormAction(): string {
 		// Nonce is verified by the caller (renderPage) before this method runs.
 		// phpcs:disable WordPress.Security.NonceVerification.Missing
 		$action = sanitize_key( wp_unslash( $_POST['usc_action'] ?? '' ) );
@@ -45,13 +51,17 @@ class RegistrationController {
 				&& false === email_exists( $email )
 				&& null === $this->invites->findPendingByEmail( $email )
 			) {
+				$rawToken = wp_generate_password( 32, false );
+
 				$this->invites->insert(
 					new Invite(
 						email:     $email,
-						token:     wp_generate_password( 32, false ),
+						token:     Invite::hashToken( $rawToken ),
 						invitedBy: get_current_user_id(),
 					)
 				);
+
+				return $this->registrationLink( $rawToken );
 			}
 		}
 
@@ -62,5 +72,17 @@ class RegistrationController {
 			}
 		}
 		// phpcs:enable WordPress.Security.NonceVerification.Missing
+
+		return '';
+	}
+
+	/**
+	 * Build the registration URL for a raw invite token.
+	 */
+	private function registrationLink( string $rawToken ): string {
+		$pageId   = (int) get_option( self::OPTION_PAGE, 0 );
+		$linkBase = $pageId > 0 ? (string) get_permalink( $pageId ) : '';
+
+		return add_query_arg( 'us_invite', rawurlencode( $rawToken ), '' !== $linkBase ? $linkBase : home_url( '/' ) );
 	}
 }