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/permission.rs

@@ -27,15 +27,15 @@ impl Permission {
     pub fn can_read(&self) -> bool {
         matches!(self, Self::Read | Self::Admin)
     }
-    
+
     pub fn can_write(&self) -> bool {
         matches!(self, Self::Write | Self::Admin)
     }
-    
+
     pub fn can_delete(&self) -> bool {
         matches!(self, Self::Delete | Self::Admin)
     }
-    
+
     pub fn can_share(&self) -> bool {
         matches!(self, Self::Share | Self::Admin)
     }