test: the app's clock never goes back in the test suite

The docker VM the suite runs in steps its wall clock back now and then (measured: 2 steps in 20 s, up to 0.5 s), so rows a test made one after another could read out of order (a files list sorted newest first, a scheduler's last runs). Every app module that took datetime from the datetime module reads a clock that never repeats or goes back; the scheduler test's own fixture for this goes.

backend/tests/conftest.py

@@ -13,6 +13,7 @@ that the migration chain applies cleanly. Each test then gets a clean database v
 tests that COMMIT (the whole point of this lane — transaction-ownership behaviour) don't leak.
 """
 
+import datetime as _dt
 import os
 import shutil
 from urllib.parse import quote
@@ -317,6 +318,41 @@ def _reset_rate_limiters():
                 obj.reset()
 
 
+class _NeverBack(_dt.datetime):
+    """The app's clock in tests: the wall clock, but never repeating and never going back — each
+    reading at least a microsecond after the last one in this process."""
+
+    last: _dt.datetime | None = None
+    clock: type[_dt.datetime] = (
+        _dt.datetime
+    )  # what it reads; a probe can put a stepping one under it
+
+    @classmethod
+    def now(cls, tz=None):
+        t = cls.clock.now(tz)
+        last = cls.last
+        if last is not None and (last.tzinfo is None) == (t.tzinfo is None) and t <= last:
+            t = last + _dt.timedelta(microseconds=1)
+        cls.last = t
+        return t
+
+
+@pytest.fixture(autouse=True)
+def _clock_never_goes_back(monkeypatch):
+    """Rows a test makes one after another are stamped in that order. The wall clock of the docker
+    VM the suite runs in steps BACK now and then (measured: 2 steps in 20 s, up to 0.5 s), and a
+    test that makes three files and reads them newest first then got them out of order. Every loaded
+    `app.*` module that took `datetime` from the datetime module reads this clock instead (a sweep,
+    as the limiter reset below does, so a new module is covered the day it lands). A database
+    default (`server_default=func.now()`) is Postgres's clock and is not covered: a test that orders
+    such rows made within a second of each other gives them distinct times itself."""
+    import sys
+
+    for name, mod in list(sys.modules.items()):
+        if (name == "app" or name.startswith("app.")) and vars(mod).get("datetime") is _dt.datetime:
+            monkeypatch.setattr(mod, "datetime", _NeverBack)
+
+
 @pytest.fixture(autouse=True)
 def _reset_log_erasures():
     """`logs._erased` is module-level and remembers an erased account for an hour of monotonic time