test: stop the suite reinstalling plugins into the real plugin-repos/ (#679)

Any test that imported web_interface.app and sent a request fired the app's
startup reconciliation, which runs against the checkout's real config.json
and plugin-repos/ and reinstalls every configured-but-missing plugin from the
live store. A full Windows run left basketball-scoreboard, calendar,
football-scoreboard, leaderboard and ledmatrix-stocks untracked in
plugin-repos/ (not gitignored) from that daemon thread.

test/conftest.py now installs an import hook that sets the app's run-once
_reconciliation_started latch as the module finishes executing, so lazy
imports, module-level imports and reloads all start disarmed.
StateReconciliation's own tests are unaffected. A regression test pins that
a request to the imported app launches no reconciliation thread.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_startup_reconciliation_disarmed.py

@@ -0,0 +1,23 @@
+"""
+No test may start the real app's startup reconciliation.
+
+web_interface/app.py reads the checkout's real config/config.json and
+plugin-repos/ at import, and its first request launches a reconciliation
+thread that reinstalls every configured-but-missing plugin from the live
+store. A full suite run on a dev checkout used to leave whole plugins
+untracked in plugin-repos/ that way. test/conftest.py disarms the run-once
+latch on every import of the module; this pins that.
+"""
+
+from unittest.mock import MagicMock, patch
+
+
+def test_a_request_to_the_imported_app_launches_no_reconciliation():
+    import web_interface.app as web_app
+
+    assert web_app._reconciliation_started is True
+
+    with patch.object(web_app, "threading", MagicMock()) as threading_mock:
+        web_app.app.test_client().get("/favicon.ico")
+
+    threading_mock.Thread.assert_not_called()