Add health check endpoint and enhance admin verification logic

- Introduced a new public `/health` endpoint to verify database connectivity and list active admin users.
- Updated `verify_admin` function to return the actual username on successful verification and handle various password hashing schemes, including legacy formats.
- Modified login logic to use the returned username for session management.
- Updated `login.html` form to support file uploads by adding `enctype` attribute.

templates/login.html

@@ -4,7 +4,7 @@
 <div class="login-card">
   <h1>Admin login</h1>
   <p class="login-hint">Only users with role <code>admin</code> can access this dashboard.</p>
-  <form method="post" action="{{ url_for('login') }}" class="login-form">
+  <form method="post" action="{{ url_for('login') }}" class="login-form" enctype="multipart/form-data">
     <label for="username">Username</label>
     <input type="text" id="username" name="username" required autofocus autocomplete="username">
     <label for="password">Password</label>