feat: add connections dashboard with background health checks

- Add /connections page showing all integrations with online/offline status
- Add background health checker that polls Evotor API every 10 minutes
- Add is_online and last_checked_at fields to evotor_connections table
- Replace Evotor navbar link with unified Connections link
- Redirect connect/disconnect flows to /connections
- Add Alembic migration for new columns

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/base.html

@@ -19,7 +19,7 @@
                 <ul class="navbar-nav ms-auto">
                     {% if user %}
                         <li class="nav-item">
-                            <a href="/evotor" class="nav-link">Эвотор</a>
+                            <a href="/connections" class="nav-link">Подключения</a>
                         </li>
                         <li class="nav-item">
                             <a href="/profile" class="nav-link"><i class="bi bi-person-circle me-1"></i>Личный кабинет</a>