Implement data-categories endpoint

[bburda]

Summary

Implement GET /apps/{id}/data-categories endpoint to return categorized groupings of data items for a given app/component. Currently returns 501 Not Implemented.

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Proposed solution

Define data categories for ROS 2 or get them from topic namespaces, or allow manifest definition for data categories.

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Additional context

• Route already registered in rest_server.cpp, needs handler implementation
• Consider making category mappings configurable via gateway_params.yaml or manifest.