Bump plugin version so the us_invites schema migration actually runs

PR #83 added kind and expires_at to us_invites and the repository started
writing them, but USC_VERSION stayed at 1.0.0-rc.2 — Plugin::boot() only
re-runs Installer/dbDelta on a version mismatch, so upgraded sites never got
the columns. Every invite insert then failed silently: nothing appeared under
Pending Invites while the admin was still shown a registration link whose
token hash was never stored.

- Version / USC_VERSION -> 1.0.0-rc.3 (triggers dbDelta on next load).
- InviteRepository::insert() returns 0 on failure instead of a stale
  insert_id, and the Invites page now shows an error notice instead of a
  dead link when creation fails (personal and group forms), including
  clearer validation messages.
- CLAUDE.md: schema changes must bump the version.

Closes #87

Co-Authored-By: Claude Fable 5 <[email protected]>

unsupervised-schedular.php

@@ -3,7 +3,7 @@
  * Plugin Name: Unsupervised Scheduler
  * Plugin URI:  https://unsupervised.ca
  * Description: Instructor/student lesson scheduling for WordPress.
- * Version:     1.0.0-rc.2
+ * Version:     1.0.0-rc.3
  * Requires at least: 6.2
  * Requires PHP: 8.1
  * Author:      Unsupervised
@@ -20,7 +20,7 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-define('USC_VERSION', '1.0.0-rc.2');
+define('USC_VERSION', '1.0.0-rc.3');
 define('USC_PLUGIN_FILE', __FILE__);
 define('USC_PLUGIN_DIR', plugin_dir_path(__FILE__));
 define('USC_PLUGIN_URL', plugin_dir_url(__FILE__));