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/user.rs

@@ -2,18 +2,18 @@ use mongodb::bson::{doc, oid::ObjectId};
 use mongodb::Collection;
 use serde::{Deserialize, Serialize};
 
-use mongodb::bson::DateTime;
 use crate::auth::password::verify_password;
+use mongodb::bson::DateTime;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct User {
     #[serde(rename = "_id", skip_serializing_if = "Option::is_none")]
     pub id: Option<ObjectId>,
-    
+
     pub email: String,
-    
+
     pub username: String,
-    
+
     pub password_hash: String,
 
     /// Password recovery phrase hash (zero-knowledge)
@@ -54,10 +54,10 @@ impl User {
     ) -> Result<Self, anyhow::Error> {
         // Import PasswordService
         use crate::auth::password::PasswordService;
-        
+
         // Hash the password
         let password_hash = PasswordService::hash_password(&password)?;
-        
+
         // Hash the recovery phrase if provided
         let recovery_phrase_hash = if let Some(phrase) = recovery_phrase {
             Some(PasswordService::hash_password(&phrase)?)
@@ -94,7 +94,7 @@ impl User {
         if !self.recovery_enabled || self.recovery_phrase_hash.is_none() {
             return Ok(false);
         }
-        
+
         let hash = self.recovery_phrase_hash.as_ref().unwrap();
         verify_password(phrase, hash)
     }
@@ -102,7 +102,7 @@ impl User {
     /// Update the password hash (increments token_version to invalidate all tokens)
     pub fn update_password(&mut self, new_password: String) -> Result<(), anyhow::Error> {
         use crate::auth::password::PasswordService;
-        
+
         self.password_hash = PasswordService::hash_password(&new_password)?;
         self.token_version += 1;
         Ok(())
@@ -111,7 +111,7 @@ impl User {
     /// Set or update the recovery phrase
     pub fn set_recovery_phrase(&mut self, phrase: String) -> Result<(), anyhow::Error> {
         use crate::auth::password::PasswordService;
-        
+
         self.recovery_phrase_hash = Some(PasswordService::hash_password(&phrase)?);
         self.recovery_enabled = true;
         Ok(())
@@ -156,13 +156,14 @@ impl UserRepository {
 
     /// Find a user by ID
     pub async fn find_by_id(&self, id: &ObjectId) -> mongodb::error::Result<Option<User>> {
-        self.collection
-            .find_one(doc! { "_id": id }, None)
-            .await
+        self.collection.find_one(doc! { "_id": id }, None).await
     }
 
     /// Find a user by verification token
-    pub async fn find_by_verification_token(&self, token: &str) -> mongodb::error::Result<Option<User>> {
+    pub async fn find_by_verification_token(
+        &self,
+        token: &str,
+    ) -> mongodb::error::Result<Option<User>> {
         self.collection
             .find_one(doc! { "verification_token": token }, None)
             .await
@@ -177,12 +178,16 @@ impl UserRepository {
     }
 
     /// Update the token version - silently fails if ObjectId is invalid
-    pub async fn update_token_version(&self, user_id: &str, version: i32) -> mongodb::error::Result<()> {
+    pub async fn update_token_version(
+        &self,
+        user_id: &str,
+        version: i32,
+    ) -> mongodb::error::Result<()> {
         let oid = match ObjectId::parse_str(user_id) {
             Ok(id) => id,
             Err(_) => return Ok(()), // Silently fail if invalid ObjectId
         };
-        
+
         self.collection
             .update_one(
                 doc! { "_id": oid },
@@ -204,7 +209,7 @@ impl UserRepository {
     /// Update last active timestamp
     pub async fn update_last_active(&self, user_id: &ObjectId) -> mongodb::error::Result<()> {
         use mongodb::bson::DateTime;
-        
+
         let now = DateTime::now();
         self.collection
             .update_one(