Remove REST handlers superseded by GraphQL resolvers

Deleted handle_activity.go, handle_favorites.go, handle_post.go, and
handle_user.go — all logic already exists in schema.resolvers.go.
Removed corresponding REST routes from main.go. Moved UserCredentials
struct (used by Login handler) into handle_auth.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.go

@@ -90,33 +90,10 @@ func main() {
 	protected := r.Group("/", store.AuthMiddlewear)
 	admin := r.Group("/", store.AuthMiddlewear, store.AdminMiddleware)
 
-	// FAVORITES
-	r.GET("/favorites", store.GetFavorites)
-	admin.POST("/favorites", store.CreateFavorite)
-
 	// ROWING
 	r.GET("/rowing", store.GetRowing)
 	admin.POST("/rowing", store.CreateRowing)
 
-	// ACTIVITIES
-	r.GET("/activity", store.GetActivity)
-	admin.POST("/activity", store.CreateActivity)
-
-	// POSTS
-	r.GET("/posts", store.GetPosts)
-	admin.POST("/posts", store.CreatePost)
-	r.GET("/posts/:id", store.GetPost)
-	admin.PUT("/posts/:id", store.UpdatePost)
-	admin.DELETE("/posts/:id", store.DeletePost)
-
-	// USERS
-	r.GET("/user/:id", store.GetUser)
-	admin.PUT("/user/:id", store.UpdateUser)
-	admin.DELETE("/user/:id", store.DeleteUser)
-	r.GET("/user", store.GetUsers)
-	admin.POST("/user", store.CreateUser)
-	admin.PATCH("/user/:id/admin", store.SetUserAdmin)
-
 	// AUTH
 	r.POST("/auth/login", store.Login)
 	r.POST("/auth/refresh", store.RefreshToken)