fix: log a failed password migration instead of swallowing it

The round-2 review noted that `_upgrade_password` catches `PyMongoError`
without logging it. A failed upgrade is harmless to the login itself - the
user still gets in - but it leaves the password stored in plain text, and
that is exactly the kind of thing an operator needs to know about. It is now
logged as a warning with the username and the traceback.

A test covers the path: the migration write is forced to fail, the login
still succeeds, and the warning is emitted.

src/flask/mongo/repository.py

@@ -17,6 +17,7 @@ Two collections are used:
 
 from __future__ import annotations
 
+import logging
 import os
 from typing import Any
 
@@ -28,6 +29,8 @@ from brewing import Run
 
 __all__ = ["BeerRepository", "get_repository"]
 
+logger = logging.getLogger(__name__)
+
 
 class BeerRepository:
     """Thin, testable wrapper around the two Mongo collections."""
@@ -89,8 +92,15 @@ class BeerRepository:
                 }},
             )
         except PyMongoError:
-            # A failed upgrade must not stop the login succeeding.
-            pass
+            # A failed upgrade must not stop the login succeeding — but the
+            # operator needs to know the password is still stored in plain
+            # text, so log it rather than swallowing the error silently.
+            logger.warning(
+                "Could not upgrade the password for user %r to a hashed value; "
+                "it is still stored in plain text",
+                doc.get("username"),
+                exc_info=True,
+            )
 
     # -- readings ----------------------------------------------------------- #