style: apply rustfmt to backend codebase

- Apply rustfmt to all Rust source files in backend/
- Fix trailing whitespace inconsistencies
- Standardize formatting across handlers, models, and services
- Improve code readability with consistent formatting

These changes are purely stylistic and do not affect functionality.
All CI checks now pass with proper formatting.

backend/src/services/mod.rs

@@ -1,14 +1,14 @@
 //! Phase 2.8 Services Module
-//! 
+//!
 //! This module contains external service integrations:
 //! - Ingredient Mapper (EU to US drug names)
 //! - OpenFDA Service (drug interactions)
 //! - Interaction Checker (combined service)
 
 pub mod ingredient_mapper;
-pub mod openfda_service;
 pub mod interaction_service;
+pub mod openfda_service;
 
 pub use ingredient_mapper::IngredientMapper;
-pub use openfda_service::OpenFDAService;
 pub use interaction_service::InteractionService;
+pub use openfda_service::OpenFDAService;