feat(auth): block reserved identifiers at sign-up

Add App\Rules\ReservedIdentifier for name + email local-part so users
cannot impersonate staff (admin, superadmin, support, root, takeone, …).
Separators are collapsed so "super_admin"/"super-admin"/"superadmin"
all match. Rule wired into RegisteredUserController@store.

email_verified_at is already absent from User::$fillable so only the
signed verification route can flip it — audited, no code change needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/Rules/ReservedIdentifier.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+/**
+ * Blocks impersonation of staff/system accounts.
+ *
+ * Applied to both `name` and the email local-part at registration so a user
+ * cannot sign up as "admin", "superadmin", "takeone", etc. — words that would
+ * let them pass themselves off as the platform or its operators to other
+ * users (comments, DMs, channel pages).
+ */
+class ReservedIdentifier implements ValidationRule
+{
+    private const RESERVED = [
+        'admin', 'administrator', 'superadmin', 'super-admin', 'super_admin',
+        'root', 'sysadmin', 'system', 'owner', 'webmaster', 'postmaster',
+        'moderator', 'mod', 'staff', 'official',
+        'support', 'help', 'helpdesk', 'contact', 'info', 'noreply', 'no-reply',
+        'security', 'abuse', 'billing',
+        'takeone', 'take-one', 'take_one', 'takeonebh', 'takeone-bh',
+        'play', 'playbh', 'playtakeone',
+        'anonymous', 'guest', 'null', 'undefined',
+    ];
+
+    public function __construct(private readonly string $kind = 'name')
+    {
+    }
+
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (! is_string($value)) {
+            return;
+        }
+
+        $candidate = $this->kind === 'email'
+            ? strtolower(trim(strstr($value, '@', true) ?: $value))
+            : strtolower(trim($value));
+
+        // Collapse separators so "super_admin", "super-admin" and "superadmin" all match.
+        $normalized = preg_replace('/[\s._\-]+/', '', $candidate) ?? $candidate;
+
+        foreach (self::RESERVED as $word) {
+            $target = preg_replace('/[\s._\-]+/', '', $word) ?? $word;
+            if ($normalized === $target) {
+                $fail($this->kind === 'email'
+                    ? 'That email address is reserved. Please use a different one.'
+                    : 'That name is reserved. Please choose a different one.');
+                return;
+            }
+        }
+    }
+}