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/thought.rs

@@ -0,0 +1,67 @@
+use sea_orm::{
+    ActiveModelTrait, ColumnTrait, DbConn, DbErr, EntityTrait, JoinType, QueryFilter, QueryOrder,
+    QuerySelect, RelationTrait, Set,
+};
+
+use models::{
+    domains::{thought, user},
+    params::thought::CreateThoughtParams,
+    schemas::thought::ThoughtWithAuthor,
+};
+
+use crate::error::UserError;
+
+pub async fn create_thought(
+    db: &DbConn,
+    author_id: i32,
+    params: CreateThoughtParams,
+) -> Result<thought::Model, DbErr> {
+    thought::ActiveModel {
+        author_id: Set(author_id),
+        content: Set(params.content),
+        ..Default::default()
+    }
+    .insert(db)
+    .await
+}
+
+pub async fn get_thought(db: &DbConn, thought_id: i32) -> Result<Option<thought::Model>, DbErr> {
+    thought::Entity::find_by_id(thought_id).one(db).await
+}
+
+pub async fn delete_thought(db: &DbConn, thought_id: i32) -> Result<(), DbErr> {
+    thought::Entity::delete_by_id(thought_id).exec(db).await?;
+    Ok(())
+}
+
+pub async fn get_thoughts_by_user(
+    db: &DbConn,
+    user_id: i32,
+) -> Result<Vec<ThoughtWithAuthor>, DbErr> {
+    thought::Entity::find()
+        .column_as(user::Column::Username, "author_username")
+        .join(JoinType::InnerJoin, thought::Relation::User.def().rev())
+        .filter(thought::Column::AuthorId.eq(user_id))
+        .order_by_desc(thought::Column::CreatedAt)
+        .into_model::<ThoughtWithAuthor>()
+        .all(db)
+        .await
+}
+
+pub async fn get_feed_for_user(
+    db: &DbConn,
+    followed_ids: Vec<i32>,
+) -> Result<Vec<ThoughtWithAuthor>, UserError> {
+    if followed_ids.is_empty() {
+        return Ok(vec![]);
+    }
+    thought::Entity::find()
+        .column_as(user::Column::Username, "author_username")
+        .join(JoinType::InnerJoin, thought::Relation::User.def().rev())
+        .filter(thought::Column::AuthorId.is_in(followed_ids))
+        .order_by_desc(thought::Column::CreatedAt)
+        .into_model::<ThoughtWithAuthor>()
+        .all(db)
+        .await
+        .map_err(|e| UserError::Internal(e.to_string()))
+}