Switch to cognito user/password authentication. Major code refactor.

app/utils/auth.py

@@ -0,0 +1,12 @@
+import base64
+import hashlib
+import hmac
+
+def calculate_secret_hash(username: str, client_id: str, client_secret: str) -> str:
+    """
+    Calculate the Cognito SECRET_HASH using HMAC SHA256 for secret-enabled clients.
+    """
+    msg = username + client_id
+    dig = hmac.new(client_secret.encode('utf-8'),
+                   msg.encode('utf-8'), hashlib.sha256).digest()
+    return base64.b64encode(dig).decode()