Add tags to points of interest

[yepzdk]

Description

Tags currently only relate to Routes via ManyToMany. Add a ManyToMany relationship between PointOfInterest and Tag, with admin UI support and optional API exposure.

Considerations

• Needs discussion on whether this is desired and how it interacts with route-level tags
• Requires a Doctrine migration for the new relationship
• Admin CRUD controller updates for both PointOfInterest and Tag
• API serialization updates if tags should be exposed on POI endpoints

Part of #35