fix(advice): an editing token, so a comment-only correction is safe too

The gravity guard caught a correction that changed the gravity, but a
COMMENT-ONLY correction leaves the gravity unchanged - so the late
thread for the old advice still matched and overwrote the fresh one.

Reproduced against the live database:

    fresh advice matched: 1
    stale thread matched: 1     <-- should be 0
    FINAL advice: 'STALE ADVICE'

Every edit now writes a fresh token onto the reading, and the advice
thread carries the token it was given.  A late thread finds a token that
no longer matches, so its write is refused.  The gravity is still
checked as well.

Verified against the live database:

    token changed: True
    fresh advice matched: 1
    stale thread matched: 0     <-- must be 0
    FINAL advice: 'FRESH ADVICE'

174 tests (up from 173).

src/flask/views/brews.py

@@ -91,6 +91,7 @@ def _start_advice(run_id: str, reading_id: str, corrected: bool = False) -> None
             points = reading.sg_points
             readings.append({
                 "_id": str(reading._id),
+                "advice_token": reading.advice_token,
                 "date": reading.date.isoformat() if reading.date else "",
                 "sg": points,
                 "sg_decimal": reading.sg_decimal,
@@ -113,10 +114,11 @@ 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.
+    # The gravity and the editing token of the reading being advised
+    # about.  The write is conditional on both, so a slow advice thread
+    # cannot overwrite a correction with advice about the old value.
     advice_sg = latest.get("sg")
+    advice_token = latest.get("advice_token")
 
     def write(read_id: str, ok: bool, text: str) -> None:
         # The client is SHARED, not built per reading.  A MongoClient
@@ -131,8 +133,11 @@ def _start_advice(run_id: str, reading_id: str, corrected: bool = False) -> None
 
         try:
             shared = get_shared_repository()
-            written = shared.set_reading_advice(read_id, text, ok,
-                                                expected_sg=advice_sg)
+            written = shared.set_reading_advice(
+                read_id, text, ok,
+                expected_sg=advice_sg,
+                expected_token=advice_token,
+            )
             if not written:
                 # The reading was corrected while the advice was being
                 # generated, so the advice no longer matches it.  That is