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/models/session.rs

@@ -1,16 +1,16 @@
-use mongodb::{
-    Collection,
-    bson::{doc, oid::ObjectId},
-};
-use futures::stream::TryStreamExt;
-use serde::{Deserialize, Serialize};
 use anyhow::Result;
+use futures::stream::TryStreamExt;
+use mongodb::{
+    bson::{doc, oid::ObjectId},
+    Collection,
+};
+use serde::{Deserialize, Serialize};
 use std::time::SystemTime;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct DeviceInfo {
-    pub device_type: String,  // "mobile", "desktop", "tablet"
-    pub os: String,           // "iOS", "Android", "Windows", "macOS", "Linux"
+    pub device_type: String, // "mobile", "desktop", "tablet"
+    pub os: String,          // "iOS", "Android", "Windows", "macOS", "Linux"
     pub browser: Option<String>,
     pub ip_address: String,
 }
@@ -21,7 +21,7 @@ pub struct Session {
     pub id: Option<ObjectId>,
     pub user_id: ObjectId,
     pub device_info: DeviceInfo,
-    pub token_hash: String,   // Hash of the JWT token
+    pub token_hash: String, // Hash of the JWT token
     pub created_at: mongodb::bson::DateTime,
     pub last_used_at: mongodb::bson::DateTime,
     pub expires_at: mongodb::bson::DateTime,
@@ -48,7 +48,7 @@ impl SessionRepository {
     ) -> Result<ObjectId> {
         let now = SystemTime::now();
         let now_bson = mongodb::bson::DateTime::from(now);
-        
+
         let expires_at = SystemTime::now()
             .checked_add(std::time::Duration::from_secs(duration_hours as u64 * 3600))
             .ok_or_else(|| anyhow::anyhow!("Invalid duration"))?;
@@ -65,11 +65,17 @@ impl SessionRepository {
             is_revoked: false,
         };
 
-        self.collection.insert_one(session, None).await?.inserted_id.as_object_id().ok_or_else(|| anyhow::anyhow!("Failed to get inserted id"))
+        self.collection
+            .insert_one(session, None)
+            .await?
+            .inserted_id
+            .as_object_id()
+            .ok_or_else(|| anyhow::anyhow!("Failed to get inserted id"))
     }
 
     pub async fn find_by_user(&self, user_id: &ObjectId) -> Result<Vec<Session>> {
-        let cursor = self.collection
+        let cursor = self
+            .collection
             .find(
                 doc! {
                     "user_id": user_id,
@@ -98,7 +104,7 @@ impl SessionRepository {
     pub async fn revoke_all_for_user(&self, user_id: &ObjectId) -> Result<()> {
         self.collection
             .update_many(
-                doc! { 
+                doc! {
                     "user_id": user_id,
                     "is_revoked": false
                 },
@@ -110,7 +116,8 @@ impl SessionRepository {
     }
 
     pub async fn cleanup_expired(&self) -> Result<u64> {
-        let result = self.collection
+        let result = self
+            .collection
             .delete_many(
                 doc! {
                     "expires_at": { "$lt": mongodb::bson::DateTime::now() }