Anchor a plug-energy test to local midnight, not the wall clock (issue #2938)

    test_nothing_derivable_before_the_first_midnight failed for 31 minutes of
    every day and passed for the other 23.5 hours -- the shape that reads as
    ordinary flakiness and gets re-run rather than fixed. @ojimpo hit it running
    the full suite at 22:10 UTC, stashed his branch to confirm it reproduced on
    clean dev, and measured the window minute by minute instead of guessing.

    The test asserts that nothing can be derived when the only snapshot was
    taken after this local midnight, and it placed that snapshot at a raw
    wall-clock offset -- now minus thirty minutes. Its comment, "taken this
    morning, after midnight", is the premise, and it is only true away from
    the boundary. For the first half hour of each local day, now minus thirty
    minutes lands before local midnight, where it is a perfectly good baseline:
    _counter_at finds it and today comes back 1.5 rather than None.

    The window is local 00:00 to 00:30, which is 22:00 to 22:30 UTC under CEST
    and 23:00 to 23:30 under CET -- it moves with DST, since the module pins
    Europe/Berlin in an autouse fixture and an outer TZ makes no difference.

    Nothing is wrong with the production code. A snapshot from before local
    midnight genuinely is a valid baseline for today, and derive_today_yesterday
    is right to treat it as one. Only the test's premise breaks at the boundary.

    The snapshot is now anchored to local_day_start(now) plus thirty minutes,
    which is the idiom the other nine snapshot writes in this file already use
    and the reason none of them can drift. Replayed across 5760 minutes covering
    four days, including both DST switch days: the old expression fails 31
    minutes per day, the new one fails none.

backend/tests/unit/services/test_plug_energy_history.py

@@ -83,7 +83,10 @@ async def test_nothing_derivable_before_the_first_midnight(db_session):
     plug = await _plug(db_session)
     now = datetime.now(timezone.utc)
     # Snapshot taken this morning, after midnight — no baseline for the day.
-    await _snapshot(db_session, plug.id, now - timedelta(minutes=30), 102.0)
+    # Anchored to the boundary, not to the wall clock: `now - 30 minutes` is
+    # before local midnight for the first half hour of every local day, which
+    # made this the only test in the file that could drift (#2938).
+    await _snapshot(db_session, plug.id, local_day_start(now) + timedelta(minutes=30), 102.0)
 
     today, yesterday = await derive_today_yesterday(db_session, plug.id, live_total_kwh=103.5)