feat(auth): password reset, forgot username, and admin user management for local accounts

- Add /forgot-password and /forgot-username page routes and templates
- Update login page label to "Username or Email" (both already accepted by backend)
- Add "Forgot password?" and "Forgot username?" links to login page
- Add POST /api/auth/forgot-username endpoint + send_forgot_username_email() utility
- Add admin endpoints: PATCH /local/{id}, POST /local/{id}/send-password-reset, POST /local/{id}/set-password
- Update admin_users.html with Edit, Password, and Reset action buttons + modals
- Add 23 tests; fix code review issues (import style, display_name clearing behaviour)
- Update docs/API.md and docs/UserGuide.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/login.html

@@ -35,8 +35,9 @@
             <form method="POST" action="/auth" class="space-y-4">
                 <input type="hidden" name="csrf_token" value="{{ csrf_token | default('', true) }}">
                 <div>
-                    <label for="username" class="block text-sm font-medium text-gray-700">Username</label>
+                    <label for="username" class="block text-sm font-medium text-gray-700">Username or Email</label>
                     <input type="text" id="username" name="username" required
+                        autocomplete="username"
                         class="mt-1 block w-full rounded-md border-gray-300 shadow-sm focus:border-blue-500 focus:ring focus:ring-blue-500 focus:ring-opacity-50">
                 </div>
 
@@ -50,9 +51,13 @@
                     Sign in
                 </button>
             </form>
-            <div class="mt-3 text-center">
+            <div class="mt-3 text-center space-x-3">
                 <a href="/forgot-password" class="text-sm text-blue-600 hover:text-blue-500">
-                    Forgot your password?
+                    Forgot password?
+                </a>
+                <span class="text-gray-300" aria-hidden="true">|</span>
+                <a href="/forgot-username" class="text-sm text-blue-600 hover:text-blue-500">
+                    Forgot username?
                 </a>
             </div>
         </div>