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/routes/evotor.py

@@ -118,22 +118,27 @@ async def evotor_callback(
         pass  # Store info is optional; token is still saved
 
     # Save or update connection
+    from datetime import datetime
     connection = db.query(EvotorConnection).filter(EvotorConnection.user_id == user.id).first()
     if connection:
         connection.access_token = access_token
         connection.store_id = store_id
         connection.store_name = store_name
+        connection.is_online = True
+        connection.last_checked_at = datetime.utcnow()
     else:
         connection = EvotorConnection(
             user_id=user.id,
             access_token=access_token,
             store_id=store_id,
             store_name=store_name,
+            is_online=True,
+            last_checked_at=datetime.utcnow(),
         )
         db.add(connection)
     db.commit()
 
-    return RedirectResponse("/evotor", 303)
+    return RedirectResponse("/connections", 303)
 
 
 @router.post("/disconnect")
@@ -150,4 +155,4 @@ async def evotor_disconnect(
         db.delete(connection)
         db.commit()
 
-    return RedirectResponse("/evotor", 303)
+    return RedirectResponse("/connections", 303)