feat: implement user follow/unfollow functionality and thought retrieval by user

- Added follow and unfollow endpoints for users.
- Implemented logic to retrieve thoughts by a specific user.
- Updated user error handling to include cases for already following and not following.
- Created persistence layer for follow relationships.
- Enhanced user and thought schemas to support new features.
- Added tests for follow/unfollow endpoints and thought retrieval.
- Updated frontend to display thoughts and allow posting new thoughts.

thoughts-backend/tests/api/follow.rs

@@ -0,0 +1,33 @@
+use super::main::{create_test_user, setup};
+use axum::http::StatusCode;
+use utils::testing::{make_delete_request, make_post_request};
+
+#[tokio::test]
+async fn test_follow_endpoints() {
+    let app = setup().await;
+    create_test_user(&app.db, "user1").await; // AuthUser is ID 1
+    create_test_user(&app.db, "user2").await;
+
+    // 1. user1 follows user2
+    let response =
+        make_post_request(app.router.clone(), "/users/user2/follow", "".to_string()).await;
+    assert_eq!(response.status(), StatusCode::NO_CONTENT);
+
+    // 2. user1 tries to follow user2 again (should fail)
+    let response =
+        make_post_request(app.router.clone(), "/users/user2/follow", "".to_string()).await;
+    assert_eq!(response.status(), StatusCode::CONFLICT);
+
+    // 3. user1 tries to follow a non-existent user
+    let response =
+        make_post_request(app.router.clone(), "/users/nobody/follow", "".to_string()).await;
+    assert_eq!(response.status(), StatusCode::NOT_FOUND);
+
+    // 4. user1 unfollows user2
+    let response = make_delete_request(app.router.clone(), "/users/user2/follow").await;
+    assert_eq!(response.status(), StatusCode::NO_CONTENT);
+
+    // 5. user1 tries to unfollow user2 again (should fail)
+    let response = make_delete_request(app.router.clone(), "/users/user2/follow").await;
+    assert_eq!(response.status(), StatusCode::NOT_FOUND);
+}