fix(advice): the remaining review findings

Four things from the earlier review of this PR.

The prompt test did not test the prompt
---------------------------------------
`test_the_advice_is_about_the_reading_that_was_saved` asserted which
dict was PASSED to the generator - not what the model was actually ASKED.
So it passed even while the prompt still said "give advice for the most
recent reading", which is the bug that made the model advise about the
wrong number while the advice was filed against the right one.

It now asserts on the prompt content: the reading is marked, its date is
named, and "most recent reading" is gone.

Verified by reverting the prompt fix - the test fails, and so does
`test_the_prompt_names_the_reading_to_advise_about`.  Two tests now
guard it, and reverting the fix breaks both.

A write that matches nothing was silent
--------------------------------------
The advice writer discarded the return value of `set_reading_advice`.  A
write that matched 0 documents - which is now the EXPECTED outcome when
the brewer corrected the reading while the advice was being generated -
was dropped without a word.  It is logged now.

A stale docstring
-----------------
The background thread "builds its own" client - it has used the shared
one since the leak was fixed.

A missing CSS class
-------------------
`advice--reading` is used on the active-beers card and the run detail
page, and was never defined, so it had no styles.

173 tests (unchanged - the new assertions were added to existing tests).

src/flask/static/css/app.css

@@ -716,6 +716,12 @@ table.data .comment { color: var(--text-muted); font-size: .84rem; }
     color: var(--amber-text);
 }
 
+/* The advice on the active-beers card and the run detail page - the
+   full text, not the clamped table cell. */
+.advice--reading {
+    margin-top: .5rem;
+}
+
 /* Advice that could not be generated - it says why. */
 .advice--failed {
     border-left-color: var(--danger);