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/health.rs

@@ -1,6 +1,6 @@
+use crate::config::AppState;
 use axum::{extract::State, response::Json};
 use serde_json::{json, Value};
-use crate::config::AppState;
 
 pub async fn health_check(State(state): State<AppState>) -> Json<Value> {
     let status = if let Ok(_) = state.db.health_check().await {
@@ -8,10 +8,10 @@ pub async fn health_check(State(state): State<AppState>) -> Json<Value> {
     } else {
         "error"
     };
-    
+
     // Use timestamp_millis for consistency with other endpoints
     let timestamp = mongodb::bson::DateTime::now().timestamp_millis();
-    
+
     Json(json!({
         "status": "ok",
         "database": status,