feat: add edit artist endpoint (section 1.4)

- Add GET/PUT /api/artists/{id} for artist settings
- Update sync to create artists table entry (library settings)
- Support partial updates for monitored, path, quality/metadata profiles
- Add e2e tests for get, edit, partial update flows

internal/api/router.go

@@ -44,6 +44,8 @@ func NewRouter(h *Handlers) *chi.Mux {
 		r.Post("/sync", h.Sync)
 
 		r.Route("/artists", func(r chi.Router) {
+			r.Get("/{id}", h.GetArtist)
+			r.Put("/{id}", h.EditArtist)
 			r.Post("/{id}/refresh", h.RefreshArtist)
 			r.Delete("/{id}", h.DeleteArtist)
 		})