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/Cargo.lock

@@ -642,6 +642,7 @@ name = "common"
 version = "0.1.0"
 dependencies = [
  "sea-orm",
+ "sea-query",
  "serde",
  "utoipa",
 ]