fix: address final-review hygiene items (pydantic config, cookie alias, argon2 exception scope)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/security.py

@@ -1,5 +1,5 @@
 from argon2 import PasswordHasher
-from argon2.exceptions import VerifyMismatchError
+from argon2.exceptions import VerificationError
 
 _hasher = PasswordHasher()
 
@@ -11,5 +11,5 @@ def hash_password(password: str) -> str:
 def verify_password(password: str, password_hash: str) -> bool:
     try:
         return _hasher.verify(password_hash, password)
-    except VerifyMismatchError:
+    except VerificationError:
         return False