fix(advice): return matched_count, and do not advise about the wrong reading

set_reading_advice returned modified_count, which is 0 both when the
guard refused the write and when the write matched but the text was
already identical.  The caller logs "the advice was not stored" on 0,
so it has to mean "the guard refused it".  It now returns
matched_count.

_start_advice fell back to the newest reading when the reading it was
asked about was not in the run.  That described the wrong number, and
the conditional write would refuse it anyway - so it now says so and
stops.

The editing token is the primary guard on the write, not the gravity.
Every edit writes a fresh token, so a comment-only correction - which
leaves the gravity unchanged - is caught too.  Verified against live
Mongo with the real repository code: the fresh advice lands, the slow
stale thread is refused, and the brewer's comment is untouched.

193 tests pass.

src/flask/mongo/repository.py

@@ -296,7 +296,13 @@ class BeerRepository:
                 "advice_at": _now_iso(),
             }},
         )
-        return result.modified_count
+        # `matched_count`, not `modified_count`.
+        #
+        # `modified_count` is 0 when the document matched but the text
+        # was already identical - which is a successful write, not a
+        # refused one.  The caller logs "the advice was not stored" when
+        # this is 0, so it has to mean "the guard refused it".
+        return result.matched_count
 
     def set_final_reading(self, beer_run_id: str, reading_date: str,
                           final: bool) -> int: