docs: add is-located-on endpoint to gateway README and changelog

Author: bburda

src/ros2_medkit_gateway/CHANGELOG.rst

@@ -53,6 +53,7 @@ Changelog for package ros2_medkit_gateway
 * ``PluginContext::get_child_apps()`` for Component-level aggregation
 * Sub-resource RBAC patterns for all collections
 * Auto-populate gateway version from ``package.xml`` via CMake
+* ``GET /apps/{id}/is-located-on`` endpoint for reverse host lookup (app to component)
 * Condition-based triggers with CRUD endpoints, SSE event streaming, and hierarchy matching
 * ``TriggerManager`` with ``ConditionEvaluator`` interface and 4 built-in evaluators (OnChange, OnChangeTo, EnterRange, LeaveRange)
 * ``ResourceChangeNotifier`` for async dispatch from FaultManager, UpdateManager, and OperationManager

src/ros2_medkit_gateway/README.md

@@ -33,6 +33,7 @@ All endpoints are prefixed with `/api/v1` for API versioning.
 - `GET /api/v1/components/{component_id}/hosts` - List apps hosted on a component
 - `GET /api/v1/components/{component_id}/depends-on` - List component dependencies
 - `GET /api/v1/areas/{area_id}/components` - List components within a specific area
+- `GET /api/v1/apps/{app_id}/is-located-on` - Get the component hosting this app
 
 ### Component Data Endpoints