feat: Refactor user and thought models to use UUIDs instead of integers

- Updated user and thought models to utilize UUIDs for primary keys.
- Modified persistence functions to accommodate UUIDs for user and thought IDs.
- Implemented tag functionality with new Tag and ThoughtTag models.
- Added migration scripts to create new tables for tags and thought-tag relationships.
- Enhanced thought creation to parse hashtags and link them to thoughts.
- Updated tests to reflect changes in user and thought ID types.

thoughts-backend/tests/api/auth.rs

@@ -20,7 +20,6 @@ async fn test_auth_flow() {
     let body = response.into_body().collect().await.unwrap().to_bytes();
     let v: Value = serde_json::from_slice(&body).unwrap();
     assert_eq!(v["username"], "testuser");
-    assert!(v["id"].is_number());
 
     let response = make_post_request(
         app.router.clone(),