fix(advice): stale advice must not overwrite a correction

The review found a race between the background advice thread and the
brewer correcting a reading.

`set_reading_advice` wrote unconditionally by id.  The advice is
generated in the background, so if the brewer corrects a reading while
the advice for the OLD gravity is still being generated, the slow thread
lands afterwards and overwrites the fresh advice with advice about a
reading that no longer exists.  The reading ends up with the final
gravity from the correction and the advice from before it.

Reproduced against the live database:

    wrote advice for 1050
    wrote advice for 1020
    late stale write matched: 0
    final sg: 1020 | final advice: 'ADVICE FOR THE CORRECTION'

The write is now conditional on the reading still holding the gravity
the advice was generated for, so a late thread matches nothing.

Also
----
`finish_reason` is now checked.  A reply that stopped at the token
ceiling is incomplete - it may be advice that stops mid-sentence - and
it was being stored as if it were the whole answer.  It is now marked
as cut off.

173 tests (up from 171).

src/flask/views/brews.py

@@ -112,20 +112,25 @@ def _start_advice(run_id: str, reading_id: str, corrected: bool = False) -> None
         log.exception("could not gather the reading for the advice")
         return
 
+    # The gravity the advice is about.  The write is conditional on the
+    # reading still holding it, so a slow advice thread cannot overwrite
+    # a correction with advice about the old value.
+    advice_sg = latest.get("sg")
+
     def write(read_id: str, ok: bool, text: str) -> None:
         # The client is SHARED, not built per reading.  A MongoClient
-        # spawns three monitor threads and pymongo does not reap them on
-        # close(), so building one per advice write accumulated three
+        # spawns three monitor threads and pymongo does not reap them
+        # on close(), so building one per advice write accumulated three
         # threads and a connection pool for every reading recorded.
         #
         # The shared client is created on first use and lives for the
         # life of the process, which is what the request-scoped
         # repository does too.
-        from mongo.repository import BeerRepository, get_shared_repository
+        from mongo.repository import get_shared_repository
 
         try:
             shared = get_shared_repository()
-            shared.set_reading_advice(read_id, text, ok)
+            shared.set_reading_advice(read_id, text, ok, expected_sg=advice_sg)
         except Exception:  # noqa: BLE001 - never let the thread die noisily
             log.exception("could not store the generated advice")