Relationship endpoints return 404 in runtime_only mode

[mfaferek93]

Bug report

Steps to reproduce

1. Start gateway in runtime_only mode
2. GET /api/v1/areas/root/related-components returns 404
3. GET /api/v1/components/root/related-apps returns 404
4. GET /api/v1/areas/{any}/hosts returns 404

Expected behavior

Relationship endpoints should return the connected entities. Areas should list their components via related-components. Components should list their apps via related-apps.

Actual behavior

All relationship navigation endpoints return 404:

GET /areas/root/related-components -> 404
GET /areas/imu/related-components  -> 404
GET /areas/joy/related-components  -> 404
GET /components/root/related-apps  -> 404
GET /areas/root/hosts              -> 404

is-located-on works for apps (200), so the relationship exists in one direction but not the reverse.

Environment

• ros2_medkit version: main branch
• ROS 2 distro: Humble
• OS: Ubuntu 22.04 on Jetson Orin Nano (aarch64)

Additional information

This breaks SOVD entity tree navigation. Client can't walk the hierarchy from area -> component -> app via standard SOVD relationship endpoints.

May be specific to runtime_only mode - in hybrid mode with manifest these might work since relationships are explicitly defined.