refactor: sweep the platform-and-sign-in epic

Split load_config into per-section readers, use contextlib.suppress and a single condition where ruff asked, remove stale noqa comments, add the SIM and RUF100 rules, share the sign_in test helper, align AD-6 with the code and record the remaining deferred findings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/tests/test_config.py

@@ -58,7 +58,7 @@ def test_process_exits_nonzero_naming_both_variables(tmp_path):
         LOG_LEVEL="loud",
         DJANGO_SETTINGS_MODULE="portal.config.settings",
     )
-    result = subprocess.run(  # noqa: S603
+    result = subprocess.run(
         [sys.executable, "manage.py", "check"],
         cwd=ROOT,
         env=env,