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/common/Cargo.toml

@@ -7,3 +7,4 @@ edition = "2021"
 serde = { workspace = true }
 utoipa = { workspace = true }
 sea-orm = { workspace = true }
+sea-query = { workspace = true }