Add comprehensive documentation and unit tests

Documentation:
- Add detailed README files for all services (auth, character, database, launcher, packet, utils, world)
- Create API documentation for the database service with detailed endpoint specifications
- Document database schema and relationships
- Add service architecture overviews and configuration instructions

Unit Tests:
- Implement comprehensive test suite for database repositories (user, character, session)
- Add gRPC service tests for database interactions
- Create tests for packet service components (bufferpool, connection, packets)
- Add utility service tests (health check, logging, load balancer, redis cache, service discovery)
- Implement auth service user tests
- Add character service tests

Code Structure:
- Reorganize test files into a more consistent structure
- Create a dedicated tests crate for integration testing
- Add test helpers and mock implementations for easier testing

tests/database-service/integration.rs

@@ -0,0 +1,12 @@
+#[cfg(test)]
+mod tests {
+    use dotenv::dotenv;
+
+    #[tokio::test]
+    async fn test_health_check() {
+        dotenv().ok();
+        // let database_url = std::env::var("DATABASE_URL").unwrap();
+        // let db = Database::new(&database_url).await;
+        // assert!(db.health_check().await);
+    }
+}