Reuse points of interest across routes

[yepzdk]

Description

Currently PointOfInterest has a ManyToOne relationship with Route (changed from ManyToMany in migration Version20241209094309). To allow reuse of POIs across multiple routes, this would need reverting to ManyToMany with a junction table, plus admin UI changes.

Considerations

• This is a significant change — needs discussion on whether it's still desired
• Requires a new Doctrine migration to revert the relationship
• Admin UI needs to handle the many-to-many association
• API serialization may need updating

Part of #35