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/migration/src/m20240101_000001_init.rs

@@ -13,10 +13,10 @@ impl MigrationTrait for Migration {
                     .if_not_exists()
                     .col(
                         ColumnDef::new(User::Id)
-                            .integer()
+                            .uuid()
                             .not_null()
-                            .auto_increment()
-                            .primary_key(),
+                            .primary_key()
+                            .default(Expr::cust("gen_random_uuid()")),
                     )
                     .col(
                         ColumnDef::new(User::Username)