Adds full support for managing model-to-model relationships in the UI and backend.
Introduces RelatedModels subpanel for linking and unlinking models in model management.
- Adds REST API routes for adding, removing, and retrieving model relationships.
- New database migration: creates model_relationships table for bidirectional links.
- New service layer (model_relationships) for relationship management.
- Updated frontend: Related models float to top of LoRA/Main grouped model comboboxes for quick access.
- Added 'Show Only Related' toggle badge to MainModelPicker filter bar
**Amended commit to remove changes to ParamMainModelSelect.tsx and MainModelPicker.tsx to avoid conflict with upstream deletion/ rewrite**
