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.

requirements.txt

@@ -3,3 +3,4 @@ psycopg2-binary>=2.9
 bcrypt>=4.0
 python-dotenv>=1.0
 gunicorn>=21.0
+passlib>=1.7