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/api/src/routers/feed.rs

@@ -0,0 +1,39 @@
+use axum::{extract::State, response::IntoResponse, routing::get, Json, Router};
+
+use app::{
+    persistence::{follow::get_followed_ids, thought::get_feed_for_user},
+    state::AppState,
+};
+use models::schemas::thought::{ThoughtListSchema, ThoughtSchema};
+
+use crate::{error::ApiError, extractor::AuthUser};
+
+#[utoipa::path(
+    get,
+    path = "/feed",
+    responses(
+        (status = 200, description = "Authenticated user's feed", body = ThoughtListSchema)
+    ),
+    security(
+        ("api_key" = []),
+        ("bearer_auth" = [])
+    )
+)]
+async fn feed_get(
+    State(state): State<AppState>,
+    auth_user: AuthUser,
+) -> Result<impl IntoResponse, ApiError> {
+    let followed_ids = get_followed_ids(&state.conn, auth_user.id).await?;
+    let thoughts_with_authors = get_feed_for_user(&state.conn, followed_ids).await?;
+
+    let thoughts_schema: Vec<ThoughtSchema> = thoughts_with_authors
+        .into_iter()
+        .map(ThoughtSchema::from)
+        .collect();
+
+    Ok(Json(ThoughtListSchema::from(thoughts_schema)))
+}
+
+pub fn create_feed_router() -> Router<AppState> {
+    Router::new().route("/", get(feed_get))
+}