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/middleware/rate_limit.rs

@@ -1,9 +1,4 @@
-use axum::{
-    extract::Request,
-    http::StatusCode,
-    middleware::Next,
-    response::Response,
-};
+use axum::{extract::Request, http::StatusCode, middleware::Next, response::Response};
 
 /// Middleware for general rate limiting
 /// NOTE: Currently a stub implementation. TODO: Implement IP-based rate limiting
@@ -18,10 +13,7 @@ pub async fn general_rate_limit_middleware(
 
 /// Middleware for auth endpoint rate limiting
 /// NOTE: Currently a stub implementation. TODO: Implement IP-based rate limiting
-pub async fn auth_rate_limit_middleware(
-    req: Request,
-    next: Next,
-) -> Result<Response, StatusCode> {
+pub async fn auth_rate_limit_middleware(req: Request, next: Next) -> Result<Response, StatusCode> {
     // TODO: Implement proper rate limiting with IP-based tracking
     // For now, just pass through
     Ok(next.run(req).await)