feat: integrate Logto OIDC for user authentication

- Add Logto OIDC integration (app/core/logto.py): CookieStorage adapter,
  create/decode session token helpers, sync_logto_user upsert
- New auth endpoints (/api/v1/auth): sign-in, callback, sign-out, me
- AuthRedirectMiddleware: protects HTML pages, redirects to /setup when
  Logto is unconfigured, to /login otherwise
- Update require_admin_auth: accepts dmarq_session cookie JWT first,
  then API key, then Bearer JWT (fully backward compatible)
- Update User model: add logto_id, username, picture, created_at, updated_at;
  make hashed_password nullable for Logto-only users; is_superuser default=True
- New Alembic migration d4e5f6a7b8c9 for the above schema changes
- Add LOGTO_ENDPOINT / LOGTO_APP_ID / LOGTO_APP_SECRET / LOGTO_REDIRECT_URI
  settings with logto_configured property
- Create login.html (Sign in with Logto button) and setup.html (step-by-step
  configuration guide)
- Update base.html: user menu with avatar/name and sign-out via Alpine.js
  fetch to /api/v1/auth/me
- Update settings.html: remove localStorage adminApiKey; session cookie is
  sent automatically by browser; add 401 → /login redirect
- Update requirements.txt: replace fastapi-users additions with logto + aiohttp
- Add test_auth.py: 18 new tests covering session tokens, CookieStorage,
  sync_logto_user, /me, /sign-in (503), /sign-out cookie clearing
- Fix test_security_extra.py: pass Request mock to require_admin_auth;
  add new test_valid_session_cookie_returns_auth_context

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/b448f585-7646-40f8-ae2d-9986c361e3fd

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/tests/test_imap_client.py

@@ -19,7 +19,6 @@ import pytest
 from app.services.imap_client import IMAPClient
 from app.services.report_store import ReportStore
 
-
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
@@ -316,9 +315,7 @@ class TestIsDmarcReportEmail:
 
     def test_dmarc_sender_matches(self):
         client = self._make_client()
-        msg = self._make_msg(
-            subject="Weekly report", from_addr="noreply@google.com"
-        )
+        msg = self._make_msg(subject="Weekly report", from_addr="noreply@google.com")
         assert client._is_dmarc_report_email(msg) is True
 
     def test_xml_attachment_matches(self):
@@ -443,9 +440,7 @@ class TestProcessAttachments:
 
     def test_bad_attachment_does_not_raise(self):
         client = self._make_client()
-        msg = email.message_from_bytes(
-            _make_email_with_attachment("report.xml", b"not xml at all")
-        )
+        msg = email.message_from_bytes(_make_email_with_attachment("report.xml", b"not xml at all"))
         # Should not raise; just returns 0
         count = client._process_attachments(msg)
         assert count == 0