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/app/src/persistence/follow.rs

@@ -0,0 +1,46 @@
+use sea_orm::{ActiveModelTrait, ColumnTrait, DbConn, DbErr, EntityTrait, QueryFilter, Set};
+
+use crate::error::UserError;
+use models::domains::follow;
+
+pub async fn follow_user(db: &DbConn, follower_id: i32, followee_id: i32) -> Result<(), DbErr> {
+    if follower_id == followee_id {
+        return Err(DbErr::Custom("Users cannot follow themselves".to_string()));
+    }
+
+    let follow = follow::ActiveModel {
+        follower_id: Set(follower_id),
+        followed_id: Set(followee_id),
+    };
+
+    follow.save(db).await?;
+    Ok(())
+}
+
+pub async fn unfollow_user(
+    db: &DbConn,
+    follower_id: i32,
+    followee_id: i32,
+) -> Result<(), UserError> {
+    let deleted_result = follow::Entity::delete_many()
+        .filter(follow::Column::FollowerId.eq(follower_id))
+        .filter(follow::Column::FollowedId.eq(followee_id))
+        .exec(db)
+        .await
+        .map_err(|e| UserError::Internal(e.to_string()))?;
+
+    if deleted_result.rows_affected == 0 {
+        return Err(UserError::NotFollowing);
+    }
+
+    Ok(())
+}
+
+pub async fn get_followed_ids(db: &DbConn, user_id: i32) -> Result<Vec<i32>, DbErr> {
+    let followed_users = follow::Entity::find()
+        .filter(follow::Column::FollowerId.eq(user_id))
+        .all(db)
+        .await?;
+
+    Ok(followed_users.into_iter().map(|f| f.followed_id).collect())
+}