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/auth/password.rs

@@ -1,10 +1,7 @@
 use anyhow::Result;
 use pbkdf2::{
-    password_hash::{
-        rand_core::OsRng,
-        PasswordHash, PasswordHasher, PasswordVerifier, SaltString
-    },
-    Pbkdf2
+    password_hash::{rand_core::OsRng, PasswordHash, PasswordHasher, PasswordVerifier, SaltString},
+    Pbkdf2,
 };
 
 pub struct PasswordService;
@@ -12,7 +9,8 @@ pub struct PasswordService;
 impl PasswordService {
     pub fn hash_password(password: &str) -> Result<String> {
         let salt = SaltString::generate(&mut OsRng);
-        let password_hash = Pbkdf2.hash_password(password.as_bytes(), &salt)
+        let password_hash = Pbkdf2
+            .hash_password(password.as_bytes(), &salt)
             .map_err(|e| anyhow::anyhow!("Failed to hash password: {}", e))?;
         Ok(password_hash.to_string())
     }
@@ -21,7 +19,8 @@ impl PasswordService {
         let parsed_hash = PasswordHash::new(hash)
             .map_err(|e| anyhow::anyhow!("Failed to parse password hash: {}", e))?;
 
-        Pbkdf2.verify_password(password.as_bytes(), &parsed_hash)
+        Pbkdf2
+            .verify_password(password.as_bytes(), &parsed_hash)
             .map(|_| true)
             .map_err(|e| anyhow::anyhow!("Password verification failed: {}", e))
     }