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/templates/settings.html

@@ -342,14 +342,19 @@ function settingsApp() {
         showCfToken: false,
         showSmtpPw: false,
 
+        // Session cookie is sent automatically by the browser (httpOnly, same-origin).
+        // No manual auth header needed for API calls from the UI.
         apiHeaders() {
-            const key = localStorage.getItem('adminApiKey') || '';
-            return { 'Content-Type': 'application/json', 'X-API-Key': key };
+            return { 'Content-Type': 'application/json' };
         },
 
         async loadSettings() {
             try {
                 const res = await fetch('/api/v1/settings', { headers: this.apiHeaders() });
+                if (res.status === 401 || res.status === 403) {
+                    window.location.href = '/login?next=/settings';
+                    return;
+                }
                 if (!res.ok) {
                     this.showFlash('Failed to load settings: ' + res.statusText, false);
                     return;