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/evotor.html

@@ -76,8 +76,8 @@
         </div>
 
         <div class="mt-3 text-center">
-            <a href="/profile" class="text-muted small">
-                <i class="bi bi-arrow-left me-1"></i>Вернуться в личный кабинет
+            <a href="/connections" class="text-muted small">
+                <i class="bi bi-arrow-left me-1"></i>Вернуться к подключениям
             </a>
         </div>