Stop a settings test depending on which tests ran before it

CI failed one test that passes here: "no such table: settings", seeding a
Setting into a database with no tables. The test was not wrong about the
product, it was asking the process-wide plugin_manager to do the work.

create_app() re-points that singleton, and eight test modules build their own
app, so it belongs to whichever ran last on the xdist worker.
_seed_plugin_settings then opens `with self._app.app_context()` on a different
app's in-memory SQLite. A sibling symptom is the registry coming back None
("'NoneType' object has no attribute 'get_all'"). Both land on whichever test
follows, never on the module that caused them, so they move with the split -
green on one machine, red on another.

The test now builds a manager for itself, sets _app and _db, and calls
_seed_plugin_settings directly. That is the routine `flask plugin upgrade-all`
runs, so what is under test has not changed, and the assertion no longer depends
on worker order. init_app is deliberately not called: it re-registers the plugin
blueprints and a second call on one app raises "The name 'backups' is already
registered".

This does NOT fix the underlying fragility, and that is a separate decision.
Running tests/test_plugins serially today gives 115 failed and 542 errors, from
the same cause; under -n auto it is nearly invisible because modules land in
separate processes. The repair is to stop create_app mutating a module-level
singleton, and it belongs in its own change rather than riding along here.

tests/test_plugins/test_endpoints_settings_declared.py

@@ -74,13 +74,30 @@ def test_the_declared_defaults_match_what_the_code_reads_with_no_row(db):
 def test_the_upgrade_pass_seeds_them_on_a_site_that_installed_earlier(app, db):
     """install/enable fire only on a state transition, so a fleet already
     running this plugin gets these rows from `flask plugin upgrade-all`, which
-    calls sync_all_plugin_settings."""
-    pm = app.extensions.get('plugin_manager')
-    if pm is None:
-        pytest.skip('no plugin manager in this build')
+    calls sync_all_plugin_settings -> _seed_plugin_settings.
+
+    This drives _seed_plugin_settings on a manager built for this test rather
+    than the process-wide plugin_manager singleton, and that is deliberate.
+    create_app() re-points that singleton, eight test modules build their own
+    app, and whatever ran last on this xdist worker owns it: the symptoms are
+    "no such table: settings" (seeding against another app's in-memory SQLite)
+    and "'NoneType' object has no attribute 'get_all'" (registry left None).
+    Both land on whichever test follows, so they move with the split - this one
+    passed locally and failed on CI for exactly that reason.
+
+    init_app is not called here either: it re-registers the plugin blueprints,
+    and twice on one app raises "The name 'backups' is already registered".
+    Setting _app and _db is all the seeding routine reads.
+    """
+    from shopdb.plugins import PluginManager
+    from plugins.endpoints.plugin import EndpointsPlugin
+
     assert Setting.query.filter_by(key=QUERY_KEY).first() is None
 
-    pm.sync_all_plugin_settings()
+    manager = PluginManager()
+    manager._app = app
+    manager._db = db
+    assert manager._seed_plugin_settings(EndpointsPlugin()) is True
 
     for key, value in ((REVOKED_KEY, False), (QUERY_KEY, True)):
         row = Setting.query.filter_by(key=key).one()