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_smtp_mailer.py

@@ -67,10 +67,10 @@ class FakeSmtpModule:
 
         return Connection()
 
-    def SMTP(self, *args, **kwargs):  # noqa: N802
+    def SMTP(self, *args, **kwargs):
         return self._connection("SMTP", args, kwargs)
 
-    def SMTP_SSL(self, *args, **kwargs):  # noqa: N802
+    def SMTP_SSL(self, *args, **kwargs):
         return self._connection("SMTP_SSL", args, kwargs)
 
     def names(self):