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.toml

@@ -17,6 +17,7 @@ members = ["api", "app", "doc", "models", "migration", "utils"]
 axum = { version = "0.8.4", default-features = false }
 tower = { version = "0.5.2", default-features = false }
 sea-orm = { version = "1.1.12" }
+sea-query = { version = "0.32.6" }                              # Added sea-query dependency
 serde = { version = "1.0.219", features = ["derive"] }
 serde_json = { version = "1.0.140" }
 tracing = "0.1.41"