Implement `GET /apps/{app-id}/belongs-to` endpoint

[bburda]

Summary

Add a new discovery endpoint that returns the Area to which a given App belongs. This is determined by finding the App's parent Component and then looking up which Area that Component belongs to.

---

Proposed solution

GET /api/v1/apps/{app-id}/belongs-to

Returns the Area that contains this App (via its parent Component's area assignment).

Path parameters:

Parameter	Type	Required	Description
app-id	string	Yes	App identifier (alphanumeric, underscores, hyphens, max 256 chars)

Request body: None

Response 200 OK:

{
  "items": [
    {
      "id": "powertrain",
      "name": "Powertrain",
      "href": "/api/v1/areas/powertrain"
    }
  ],
  "_links": {
    "self": "/api/v1/apps/{app-id}/belongs-to",
    "app": "/api/v1/apps/{app-id}"
  }
}

The items array will contain exactly 0 or 1 area. It may be empty if:

• The App has no parent Component (component_id is empty)
• The parent Component has no assigned Area

Response 400 Bad Request:

Returned when the app-id format is invalid.

{
  "error_code": "invalid-parameter",
  "message": "Invalid app ID",
  "parameters": {
    "app_id": "bad id!"
  }
}

Response 404 Not Found:

Returned when no App with the given ID exists.

{
  "error_code": "entity-not-found",
  "message": "App not found",
  "parameters": {
    "app_id": "nonexistent_app"
  }
}

---

Additional context

Logic flow

1. Validate app-id format
2. Look up the App via DiscoveryManager::get_app(app_id) → 404 if not found
3. Get app.component_id → if empty, return empty items array
4. Look up the Component via DiscoveryManager::get_component(component_id)
5. Get component.area → if empty, return empty items array
6. Look up the Area via DiscoveryManager::get_area(area_id) to get its name
7. Return the Area as an EntityReference

Existing patterns to follow

• Follow the same handler structure as handle_app_depends_on and handle_get_hosts in discovery_handlers.cpp
• Use the standard relationship response envelope

Route registration

Register in rest_server.cpp::setup_routes():

srv->Get((api_path("/apps") + R"(/([^/]+)/belongs-to$)"), handler);

Handler method

Add to DiscoveryHandlers:

void handle_app_belongs_to(const httplib::Request& req, httplib::Response& res);

Tests

• Unit test: verify correct area returned for app with known component→area chain
• Unit test: verify empty items when app has no component
• Unit test: verify empty items when app's component has no area
• Unit test: 404 for nonexistent app
• Integration test: launch demo nodes, query the endpoint