test: the cookie signer and test modules read the never-back clock too

The docker VM's clock steps back: itsdangerous refused a cookie signed in the next second (a
random 401 right after a sign-in), and a test compared an app stamp with the raw wall clock.

backend/tests/conftest.py

@@ -337,20 +337,45 @@ class _NeverBack(_dt.datetime):
         return t
 
 
+_last_signature_second = [0]
+
+
+def _signature_second_never_back(self) -> int:
+    """The session cookie's signing clock (itsdangerous, whole seconds), never going back: it refuses
+    a signature from the future, so a step back across a second boundary refused the cookie a
+    sign-in had just set. Read through the module at call time, so a probe can step it."""
+    import itsdangerous.timed
+
+    t = max(int(itsdangerous.timed.time.time()), _last_signature_second[0])
+    _last_signature_second[0] = 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
+    as the limiter reset below does, so a new module is covered the day it lands) — and so does every
+    test module, so a test comparing a stamp the app wrote with "now" reads the same clock. The
+    session cookie's signature has a clock of its own (`_signature_second_never_back`). 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
 
+    from itsdangerous import TimestampSigner
+
+    tests_dir = os.path.dirname(os.path.abspath(__file__))
     for name, mod in list(sys.modules.items()):
-        if (name == "app" or name.startswith("app.")) and vars(mod).get("datetime") is _dt.datetime:
+        if vars(mod).get("datetime") is not _dt.datetime:
+            continue
+        mod_file = getattr(mod, "__file__", None) or ""
+        if name == "app" or name.startswith("app.") or mod_file.startswith(tests_dir + os.sep):
             monkeypatch.setattr(mod, "datetime", _NeverBack)
+    # Per test: a cookie never outlives its test, and a probe's stepped clock must not carry over.
+    _last_signature_second[0] = 0
+    monkeypatch.setattr(TimestampSigner, "get_timestamp", _signature_second_never_back)
 
 
 @pytest.fixture(autouse=True)