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/brewing.py

@@ -156,6 +156,10 @@ class Reading:
     # Mongo, so the brewer's own comment is never overwritten.
     advice: str = ""
     advice_ok: bool = True
+    # Marks this generation of the reading.  Every edit writes a fresh
+    # one, so an advice thread working on an earlier version of the
+    # reading can be told apart from the current one.
+    advice_token: str = ""
 
     @property
     def sg_decimal(self) -> float | None:
@@ -177,6 +181,7 @@ class Reading:
             _id=str(doc.get("_id") or ""),
             advice=(doc.get("advice") or "").strip(),
             advice_ok=bool(doc.get("advice_ok", True)),
+            advice_token=str(doc.get("advice_token") or ""),
         )