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/handlers/interactions.rs

@@ -34,10 +34,12 @@ pub async fn check_interactions(
     if request.medications.len() < 2 {
         return Err(StatusCode::BAD_REQUEST);
     }
-    
-    let interaction_service = state.interaction_service.as_ref()
+
+    let interaction_service = state
+        .interaction_service
+        .as_ref()
         .ok_or(StatusCode::SERVICE_UNAVAILABLE)?;
-    
+
     match interaction_service
         .check_eu_medications(&request.medications)
         .await
@@ -46,7 +48,7 @@ pub async fn check_interactions(
             let has_severe = interactions
                 .iter()
                 .any(|i| matches!(i.severity, InteractionSeverity::Severe));
-            
+
             Ok(Json(InteractionResponse {
                 interactions,
                 has_severe,
@@ -76,9 +78,11 @@ pub async fn check_new_medication(
     State(state): State<AppState>,
     Json(request): Json<CheckNewMedicationRequest>,
 ) -> Result<Json<InteractionResponse>, StatusCode> {
-    let interaction_service = state.interaction_service.as_ref()
+    let interaction_service = state
+        .interaction_service
+        .as_ref()
         .ok_or(StatusCode::SERVICE_UNAVAILABLE)?;
-    
+
     match interaction_service
         .check_new_medication(&request.new_medication, &request.existing_medications)
         .await